Organic nitrates as ignition enhancers

ABSTRACT

A diesel fuel composition comprising an organic nitrate is described. The organic nitrate may be a terpene nitrate. Methods of using an organic nitrate for achieving a desired cetane number, and uses of organic nitrates for the purpose of reducing the ignition delay of the fuel and/or for increasing its cetane number to a defined level are also described, as are methods of operating a compression ignition engine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of European PatentApplication No. 11195433.5, filed on Dec. 22, 2011, the disclosure ofwhich is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate to a method of improvingdiesel fuels, and in particular to the use of organic nitrates asadditives in a diesel fuel composition to give improvements in fuelcombustion and cetane number.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, whichmay be associated with exemplary embodiments of the present invention.This discussion is believed to assist in providing a framework tofacilitate a better understanding of particular aspects of the presentinvention. Accordingly, it should be understood that this section shouldbe read in this light, and not necessarily as admissions of any priorart.

The cetane number of a fuel composition is a measure of its ease ofignition and combustion. With a lower cetane number fuel a compressionignition (diesel) engine tends to be more difficult to start and may runmore noisily when cold; conversely a fuel of higher cetane number tendsto impart easier cold starting, to lower engine noise, to alleviatewhite smoke (“cold smoke”) caused by incomplete combustion after.

There is a general preference, therefore, for a diesel fuel compositionto have a high cetane number, a preference which has become stronger asemissions legislation grows increasingly stringent, and as suchautomotive diesel specifications generally stipulate a minimum cetanenumber. To this end, many diesel fuel compositions contain ignitionimprovers, also known as cetane boost additives or cetane (number)improvers/enhancers, to ensure compliance with such specifications andgenerally to improve the combustion characteristics of the fuel.

Organic nitrates have been known for some time as ignition accelerantsin fuels, and some are also known to increase the cetane number ofdiesel fuels. Such organic nitrates generally include short- andmedium-chain linear and branched alkanols and nitrates of cycloalkanols,such as those described in U.S. Pat. No. 4,479,905.

A commonly used diesel fuel ignition improver is 2-ethylhexyl nitrate(2-EHN), which operates by shortening the ignition delay of a fuel towhich it is added. However, 2-EHN is also a radical initiator, and canpotentially have an adverse effect on the thermal stability of a fuel.Poor thermal stability in turn results in an increase in the products ofinstability reactions, such as gums, lacquers and other insolublespecies. These products can block engine filters and foul fuel injectorsand valves, and consequently can result in loss of engine efficiency oremissions control.

The organic nitrates described in the prior art as combustion improversand/or cetane number improvers have a series of disadvantages,especially lack of thermal stability, excessively high volatility andinsufficient efficacy. However, it may be expected that by decreasingthe volatility of a cetane enhancer, e.g. by using a molecule of highermolecular weight, its efficacy as a combustion improver and/or cetanenumber improver may then decline.

There are also health and safety concerns regarding the use of 2-EHN,which is a strong oxidising agent and is also readily combustible in itspure form. It can also be difficult to store in concentrated form as ittends to decompose, and so is prone to forming potentially explosivemixtures. Furthermore, it has been noted that 2-EHN functions mosteffectively under mild engine conditions.

These disadvantages, taken together with the often significant cost ofincorporating 2-EHN as an additive into a fuel composition, mean that itwould be generally desirable to reduce or eliminate the need for 2-EHNand other known cetane number improvers in diesel fuel compositions,whilst at the same time maintaining acceptable combustion properties.

WO2008/000778 describes one such approach to reducing the amount of anignition enhance required in a diesel fuel by using a Fischer-Tropschderived fuel component, in a fuel composition containing an ignitionimprover, which acts to enhance the effect of the ignition improver andthus reduce the amount required to achieve the same desired cetanenumber.

WO2006/067234 relates to the use of fatty acid alkyl esters (FAAEs) indiesel fuels to increase the cetane number.

Thus, it is desirable to overcome or alleviate at least one of theproblems associated with the prior art.

SUMMARY OF THE INVENTION

Embodiments of the invention provide alternative organic nitrates whichare effective as combustion improvers or cetane number improvers.Embodiments of the invention also provide alternative organic nitrateswhich have similar or lower volatility than known cetane numberimprovers, or which meet acceptable safety levels for use in commercialdiesel fuels. In addition, embodiments of the invention providealternative organic nitrates for use as ignition/combustion improversand that are most cost-effective and/or more convenient to manufacturethan known organic nitrate cetane number improvers. Also, embodiments ofthe invention provide alternative organic nitrates for use as cetanenumber enhancers that work well under harsh engine conditions (forexample, some known cetane enhancers may undesirably over-advancecombustion. Embodiments of the invention further provide cetaneenhancers derived from renewable (or waste) feedstocks or by-products.Additionally, embodiments of the invention provide methods for producingorganic nitrates useful as cetane number improvers by nitration ofcorresponding organic alcohols.

Surprisingly, it has been found that certain long chain linear organicnitrates, certain cyclic terpene organic nitrates, and certain nitratesderived from fatty alcohols and fatty acid alkyl esters can serve toreduce the ignition delay and/or as effective cetane number improvers indiesel fuels.

Accordingly, in a first aspect of the invention, there is provided adiesel fuel composition for use in a compression ignition engine, whichcomprises an organic nitrate selected from the group consisting of:

a cyclic nitrate of Formula (4):

wherein each of R₁ to R₉ is independently selected from H or C₁-C₆alkyl, or nitrate(—ONO₂), wherein optionally one of R₄ and R₅ forms anoptionally substituted alkylene bridge with one of R₈ and R₉, which maybe substituted by one or more C₁-C₆ alkyl, and/or nitrate(—ONO₂);wherein at least one of R₁ to R₉ is not H, and provided that no morethan one R₂ to R₉ comprises a nitrate group.

In one embodiment, the organic nitrate has the effect of increasing thecetane number of fuel, such as to a desired or target cetane number.

One or more additional organic nitrates may be used in the diesel fuelcomposition. Embodiments of the present invention also define theaddition organic nitrates.

In one embodiment, the diesel fuel composition has a cetane number of 40or more, 50 or more, 60 or more, or 70 or more.

In another aspect of the invention, there is provided a method forreducing the ignition delay and/or increasing the cetane number of adiesel fuel composition, which method comprises adding to thecomposition an amount of an organic nitrate according to the invention.

The method may involve increasing the cetane number of the diesel fuelcomposition to achieve a target cetane number. In some embodiments, themethod may involve adding one or more additional organic nitrate to thefuel composition.

In one embodiment, the method may further be for reducing the amount of2-ethylhexyl nitrate (2-EHN) or any other known cetane enhancer in thediesel fuel composition to achieve the target cetane number.

A further aspect of the invention is directed to the use of an organicnitrate in a diesel fuel composition for the purpose of reducing theignition delay (ID) of the diesel fuel composition, wherein the organicnitrate is as defined herein.

The diesel fuel composition of this or any other aspect may comprise abiofuel, and optionally may comprise FAAEs, such as FAMEs.

The organic nitrate may be present in the diesel fuel composition at aconcentration of: (a) between 0.025% and 2.0% w/w; (b) between 0.05% and1.0% w/w; or (c) one of 0.05% w/w, 0.1% w/w, 0.5% w/w or 1.0% w/w; basedon the total weight of the fuel composition.

In a preferred embodiment, the organic nitrate is selected from thegroup consisting of bornyl nitrate, fenchyl nitrate, menthly nitrate,and any combination thereof.

The embodiments of the present invention may additionally oralternatively be used to adjust any property of the fuel compositionwhich is equivalent to or associated with cetane number, for example, toimprove the combustion performance of the fuel composition, e.g. toshorten ignition delays (i.e. the time being fuel injection and ignitionin a combustion chamber during use of the fuel), to facilitate coldstarting or to reduce incomplete combustion and/or associated emissionsin a fuel-consuming system running on the fuel composition) and/or toimprove fuel economy or exhaust emissions generally.

Accordingly, in further aspects of the invention there is provided amethod or use of an organic nitrate in a diesel fuel composition forimproving the fuel economy of an engine into which the fuel compositionis or is intended to be introduced, or of a vehicle powered by such anengine, wherein the organic nitrate is defined herein.

In yet another aspect of the invention there is provided a method forthe preparation of a diesel fuel composition having a target cetanenumber for use in a compression ignition engine. The method comprisesadding an organic nitrate, as defined elsewhere herein, to the dieselfuel composition; and blending the organic nitrate with the diesel fuelcomposition to provide a diesel fuel composition having the targetcetane number.

Still yet another aspect of the invention relates to a method ofoperating a compression ignition engine and/or a vehicle which ispowered by such an engine, which method involves introducing into acombustion chamber of the engine a diesel fuel composition as definedelsewhere herein, or as obtained by the uses and methods of theinvention.

Other features of embodiments of the present invention will becomeapparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by the accompanying drawings inwhich:

FIG. 1 depicts the structures of exemplary embodiments of organicnitrates according to aspects of the invention;

FIG. 2 illustrates the reduction in ignition delay (ID) as a percentagefor a diesel base fuel comprising certain embodiments of the organicnitrates according to aspects of the invention;

FIG. 3 illustrates the results of derived ignition quality (DIQ) studiesfor a diesel base fuel comprising an exemplary embodiment of organicnitrates according to aspects of the invention under certain combustionconditions;

FIG. 4 is a graph illustrating correlations between the measuredignition delay at the various combustion conditions (a01 to a11—see Key)against the organic nitrate used as a cetane enhancer in a diesel fuelcomposition according to the invention. The results are shown againstmolecular weight of the cetane enhancer: bornyl nitrate (1), menthylnitrate (2), 1,10-decyl dinitrate (3), oleyl nitrate (4), hexadecylnitrate (5), nitro-substituted methyl oleate (6), and nitro-substitutedethyl abietate (7); and

FIG. 5 is a differential scanning calorimetry (DSC)/thermogravimetricanalysis (TGA) plot illustrating the thermal decomposition of 1,10-decyldinitrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In order to assist with the understanding of the invention several termsare defined herein.

The terms “cetane (number) improver” and “cetane (number) enhancer” areused interchangeably to encompass any component that, when added to afuel composition at a suitable concentration, has the effect ofincreasing the cetane number of the fuel composition relative to itsprevious cetane number under one or more engine conditions within theoperating conditions of the respective fuel or engine. The term cetanenumber improvers/enhancers of the invention are organic nitrates asdescribed herein. As used herein, a cetane number improver or enhancermay also be referred to as a cetane number increasing additive/agent orthe like.

In one embodiment, the cetane number of a fuel composition may bedetermined in any known manner, for instance using the standard testprocedure ASTM D613 (ISO 5165, IP 41) which provides a so-called“measured” cetane number obtained under engine running conditions. In apreferred embodiment, the cetane number may be determined using the morerecent and accurate “ignition quality test” (IQT; ASTM D6890, IP 498),which provides a “derived” cetane number based on the time delay betweeninjection and combustion of a fuel sample introduced into a constantvolume combustion chamber. This relatively rapid technique can be usedon laboratory scale (ca 100 ml) samples of a range of different fuels.Alternatively, cetane number may be measured by near infraredspectroscopy (NIR), as for example described in U.S. Pat. No. 5,349,188.This method may be preferred in a refinery environment as it can be lesscumbersome than for instance ASTM D613. NIR measurements make use of acorrelation between the measured spectrum and the actual cetane numberof a sample. An underlying model is prepared by correlating the knowncetane numbers of a variety of fuel samples with their near infraredspectral data.

In some embodiments, the methods/uses encompass adding a cetane enhanceraccording to aspects of the present invention to a fuel composition soas to adjust the cetane number or to achieve or reach a desired targetcetane number. In the context of the embodiments of this invention, to“reach” a target cetane number can also embrace exceeding that number.Thus, the target cetane number may be a target minimum cetane number.

In one embodiment, the present invention results in a fuel compositionwhich has a derived cetane number (IP 498) of 50 or greater, morepreferably of 51, 52, 53, 54 or 55 or greater. For example, in someembodiments, the resultant fuel composition may have a cetane number of60 or greater, 65 or greater or even 70 or greater.

Embodiments of the present invention may additionally or alternativelybe used to adjust any property of the fuel composition which isequivalent to or associated with cetane number, for example, to improvethe combustion performance of the fuel composition, e.g. to shortenignition delays (i.e. the time between fuel injection and ignition in acombustion chamber during use of the fuel), to facilitate cold startingor to reduce incomplete combustion and/or associated emissions in afuel-consuming system running on the fuel composition) and/or to improvefuel economy or exhaust emissions generally.

In accordance with embodiments of the invention, therefore, cetanenumber improvers also encompass additives that increase thecombustability of the fuel to which it is added and, as such, decreasethe ignition delay. Therefore, as used herein, an organic nitrate thatincreases the combustability (i.e. a “combustion enhancer/improver”)and/or decreases the ignition delay (i.e. an “ignitionenhancer/improver”) is also considered to be a cetane number improver orenhancer.

Cetane number improvers of the invention may be used to increase thecetane number of a fuel composition. As used herein, an “increase” inthe context of cetane number embraces any degree of increase compared toa previously measured cetane number under the same or equivalentconditions. Thus, in a preferred embodiment, the increase is compared tothe cetane number of the same fuel composition prior to incorporation ofthe cetane number increasing (or improving) component or additive.Alternatively, the cetane number increase may be measured in comparisonto an otherwise analogous fuel composition (or batch or the same fuelcomposition) that does not include the cetane number enhancer of theinvention. Alternatively, an increase in cetane number of a fuelrelative to a comparative fuel may be inferred by a measured increase incombustability or a measured decrease in ignition delay for thecomparative fuels.

The increase in cetane number (or the decrease in ignition delay, forexample) may be measured and/or reported in any suitable manner, such asin terms of a percentage increase or decrease. By way of example, thepercentage increase or decrease may be at least 1%, such as at least 2%.In one embodiment, the percentage increase in cetane number or decreasein ignition delay is at least 5%, at least 10%, at least 15% or at least20%. In some embodiments, the increase in cetane number or decrease inignition delay may be at least 25%, at least 30%. However, it should beappreciated that any measurable improvement in cetane number or ignitiondelay may provide a worthwhile advantage, depending on what otherfactors are considered important, e.g. availability, cost, safety and soon.

The engine in which the fuel composition of the invention is used may beany appropriate engine. Thus, where the fuel is a diesel or biodieselfuel composition, the engine is a diesel or compression ignition engineLikewise, any type of diesel engine may be used, such as a turbo chargeddiesel engine, provided the same or equivalent engine is used to measurefuel economy with and without the cetane number increasing component.Similarly, the invention is applicable to an engine in any vehicle.Generally, the cetane number improvers of the invention are suitable foruse over a wide range of engine working conditions. However, someorganic nitrates of the invention may provide optimal effects under aparticular narrow range of engine working conditions, such as under mildconditions and more suitably under harsh conditions.

Cetane Number Enhancers/Ignition Improvers

Cetane number enhancers are known and commercially available, and mayalso be known (in the context of diesel fuels) as “cetane (number)improvers”, “combustion improvers” and “ignition improvers” etc. aspreviously described.

Cetane enhancers are often added to diesel fuels, at additive levels(typically 0.1 to 2.0% w/w), to improve the combustion properties of thefuel. They function to reduce the ignition delay, i.e. the periodbetween the time of injection of the fuel and the start of combustion(ignition). This, in turn, leads to better engine performance, forexample, in terms of higher fuel efficiency, lower emissions, reducedcombustion noise and improved cold starting. Addition of a cetaneenhancer to a diesel fuel allows the point in the diesel cycle at whichheat is released to be advanced, which results in improved thermodynamicefficiency (maximum efficiency at about 10° after top dead centre.

Although there are various explanations of the working of cetaneenhancers, such as their effect in increasing the heating rate of thefuel, it is generally accepted that they act as sources ofchain-initiating radicals.

The cetane number (CN) of a fuel is defined by reference to the ignitionproperties of standard mixtures of n-hexadecane (cetane, CN=100) and2,2,4,4,6,8,8-hepta-methylnonane (CN=15). A fuel with a high CN has ashort ignition delay. Typically, molecules with high octane numbers,which confer a resistance to spontaneous ignition in gasoline sparkignition engines, have low cetane numbers. The addition of small amountsof cetane enhancers to a diesel fuel may, therefore, result in improvedfuel properties based on the shorter ignition delay.

Known cetane number enhancers include: a) certain organic nitrates (e.g.isopropyl nitrate, 2-ethylhexyl nitrate (2-EHN), cyclohexyl nitrate, andmethoxyethyl nitrate); b) organic peroxides and hydroperoxides (e.g.di-tert-butyl peroxide); and c) organic peracids and peresters. The mostcommonly used cetane enhances are dialkylperoxides (ROOR,di-t-butylperoxide) and organic nitrates (R—ONO₂), of which the mostimportant is 2-ethylhexylnitrate (2-EHN). European consumption of 2-EHNgrew from 75 kt/a to 101 kt/a from 2000 to 2008, and an average annualgrowth of approximately 3.5% has been predicted from 2008 to 2013.

The consumption of 2-EHN in North America (USA: 7.2 kt/a in 2008) ismuch lower than in Europe.

2-EHN is produced industrially by the nitration of 2-ethylhexanol, andin Europe this consumes almost a quarter of the production of thisalcohol. The nitration of the alcohol involves reaction with a 1/1mixture of undiluted nitric and sulfuric acids (using stoichiometricamounts of alcohol and nitric acid).

There are some safety concerns surrounding the production, transport anduse of 2-EHN. For example, it is conceivable that 2-EHN drums exposed tohigh temperatures during transport could be subject to runawaydecomposition reaction. The low flash point of 2-EHN (76° C.) is also aconcern. Furthermore, the auto-ignition temperature of 2-EHN of 130° C.is lower than normal hydrocarbons. There have been very many studies ofthe thermal stability of 2-EHN (e.g. Pritchard (1989), Combustion andFlame, 75, 415; Bornemann & Scheidt (2000), F. Int. J. Chem. Kinetics,34, 34; and Zeng et al. (2008), J. Thermal Analysis & calorimetry, 91,359).

Accordingly, it was desired to identify further alternative cetanenumber improvers, which may provide benefits over the known cetaneenhancer. Any useful benefit may be achieved, such as in relation totheir synthesis, storage, transportation; or in use, e.g. under certainoperating conditions or in certain diesel fuels. Particular benefits ofthe invention are directed to one or more of the following: increasedstability under storage and transport conditions; at least equal andmore suitably greater effectiveness as a cetane enhancer; organicnitrates derivable from renewable feedstocks or waste streams;effectiveness under harsh engine conditions; and effectiveness atcomponent or more suitably at additive concentration levels.

Thus, embodiments of the present invention provide alternative organicnitrates for use as cetane number improvers in diesel fuels andoptionally for achieving one or more associated benefit.

The cetane number improver according to aspects of the invention may beselected from nitrated:

(a) terpene alcohols, particularly monoterpene alcohols and mostpreferably monocyclic and bicyclic molecules including pinenes, such asborneol, fenchol and menthol;

(b) fatty alcohols, particularly obtained by hydrogenation of fatty acidesters (e.g. the synthetic alcohol mixture, Neodol 23, derived from theSHF process)

(c) unsaturated fatty esters, particularly FAMEs, such as methyl oleate;

(d) tall oil derived resin esters, particularly abietate esters such asethyl abietate; and

(e) long-chain linear alkanols, particularly linear C₁₀-C₁₈ alkanols anddiols, such as 1-octanol, 1,10,-decanediol, 1-dodecenol, 1-tridecanol,1-tetradecanol, 1-hexadecanol, 1-octadecanol; and any combinationthereof.

Accordingly, the cetane number improver has one or two nitrate (NO₃)groups in place of the hydroxyl groups in the above-mentioned compounds.

With the exception of 1,10,-decanediol, generally the alcohol feedstocksare commercially available at scales of at least 10 kt/a, which wouldpotentially be compatible with production of new useful cetane enhancersfor worldwide consumption. 1,10-decanediol may be obtained by thehydrogenation of sebacic acid (1,8-octanedicarboxylic acid), whichitself is produced via the alkali fusion of ricinoleic acid, the majorconstituent of castor oil.

Thus, in accordance with a first embodiment of the invention, the cetanenumber improver of the invention has the Formula (1): R—ONO₂, wherein Ris a terpene or an oxygenated (saturated) terpene. Optionally, theterpene may be natural or substituted by up to three (e.g. 1, 2 or 3)C₁-C₆ alkyl groups or a further nitrate (—ONO₂) group.

Terpenes are classified according to the number of units of the basicstructure methylbuta-1,3-diene or isoprene, which make up the terpene.Monoterpenes contain two isoprene units and are generally considered tohave the chemical formula C₁₀H₁₆. However, monoterpenes are particularlysensitive to oxygenation at the carbon-carbon double bond and somonoterpenes are typically saturated hydrocarbons lacking thecarbon-carbon double bond. These oxygenated, saturated molecules aresometimes also referred to as monoterpenes, and are encompassed by theterm “monoterpene” as used in the context of this invention.Monoterpenoid is another term that is understood to include themonoterpenes and other related compounds having the monoterpeneskeleton, and such monoterpenoid structures are also encompassed withinthe definition of monoterpene used herein.

Monoterpenes may be acyclic such as myrcene and ocimene or cyclic suchas limonene and pinene. In one embodiment, the terpene is a monocyclicor a bridged-monocyclic (i.e. bicyclic) alkyl.

In one embodiment, the cetane enhancer of the invention has the Formula(2): C₁₀H₁₆X—ONO₂, wherein X is selected from H, C₁-C₆ alkyl and ONO₂.In a preferred embodiment, X is selected from H, methyl, ethyl and ONO2;and still more preferably, X is H. Most preferably, R of Formula (1) isa monoterpene selected from menthyl, fenchyl and bornyl, which mayoptionally be substituted by X as defined above.

According to embodiments of the invention, therefore, the cetane numberimprover may be a compound of Formula (3):

wherein each of R₁ to R₉ is independently selected from H or C₁-C₆alkyl, or nitrate(—ONO₂), wherein optionally two of R₁ to R₉ may beconnected together to form a bridge, which may be substituted by one ormore C₁-C₆ alkyl, and/or nitrate(—ONO₂); provided that no more than 1 R₁to R₉ comprises a nitrate group. In one embodiment, R₁ to R₉ areindependently selected from H or C₁-C₆ alkyl, wherein optionally one ofR₄ and R₅ forms an optionally substituted alkylene bridge with one of R₈and R₉. Preferably at least one of R₁ to R₉ is not H; more preferably 1,2, 3, 4 or 5 of R₁ to R₉ is not H.

The C₁-C₆ alkyl may be straight chain (i.e. linear) or branched chain,wherein the number 1 to 6 refers to the total number of carbon atoms inthe group. In one embodiment, C₁-C₆ alkyl radicals include methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, sec-butyl,n-pentyl, isopentyl, tert-pentyl, sec-pentyl, n-hexyl, 2-ethylbutyl, and2,3-dimethylbutyl.

In accordance with a preferred embodiment, R₁ to R₉ are independentlyselected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyland tert-butyl, wherein optionally one of R₄ and R₅ forms an optionallysubstituted alkylene bridge with one of R₈ and R₉. In one embodiment,the alkylene bridge has the formula —(CR_(a)R_(b))_(n)—, wherein R_(a)and R_(b) are independently selected from H, methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2. In apreferred embodiment, R_(a) and R_(b) are independently selected from H,methyl and ethyl, and n is 1; and yet more preferably selected from Hand methyl, and n is 1.

In a preferred embodiment, R₁, R₆ and R₇ are H. In another preferredembodiment R₁, R₆ and R₇ are H, one of R₄ and R₅ is H, one of R₈ and R₉is H, and the other of R₄ and R₅, and R₈ and R₉ is selected from methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl or they areconnected together to form an alkylene bridge of the formula—CR_(a)R_(b)—, wherein R_(a) and R_(b) are defined above. In oneparticularly preferred embodiment, one of R₄ and R₅ is H, and the otheris methyl; and one of R₈ and R₉ is H, and the other is isopropyl;wherein R₁, R₂, R₃, R₆ and R₇ are as defined in any of the aboveembodiments; preferably wherein R₁, R₆ and R₇ are H, and R₂ and R₃ areas defined in any of the above embodiments; and most preferably whereinR₁, R₂, R₃, R₆ and R₇ are H.

In yet another particularly preferred embodiment of Formula (3) or (3A),one of R₄ and R₅ is H, one of R₈ and R₉ is H or methyl, and the other ofR₄ and R₅ and of R₈ and R₉ are connected together to form an alkylenebridge of the formula —CR_(a)R_(b)—, wherein R_(a) and R_(b) are H ormethyl. Beneficially in this preferred embodiment, one of R₈ and R₉ ismethyl, R₁, R₆ and R₇ are H, R_(a) and R_(b) are H or methyl, and R₂ andR₃ are as defined in any of the above embodiments. In one preferredgroup of compounds of this embodiment, at least two of R_(a), R_(b), R₂and R₃ are methyl. For example, in one embodiment R_(a) and R_(b) aremethyl, and in another embodiment R₂ and R₃ are methyl.

Accordingly, in another preferred embodiment of the invention, thecetane number improver is defined by Formula (4):

wherein each of R₁ to R₉ is as defined in connection with Formula (3).In one embodiment, at least one of R₁ to R₉ is not H, and morepreferably, 1, 2, 3, 4 or 5 of R₁ to R₉ is not H. In a preferredstructure of Formula (4) or Formula (4A), R₈ and R₄ are connectedtogether to form an alkylene bridge of the formula —(CR_(a)R_(b))_(n)—,wherein R_(a) and R_(b) are independently selected from H, methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1or 2. More preferably the alkylene bridge has the formula —CR_(a)R_(b)—,wherein R_(a) and R_(b) are H or methyl. Also preferred are compounds ofFormula (4) and Formula (4A) in which R₁, R₆ and R₇ are H. Still morepreferred are compounds of Formula (4) and Formula (4A) wherein R₁, R₆and R₇ are H and at least two of R_(a), R_(b), R₂ and R₃ are methyl. Inone particular embodiment R_(a) and R_(b) are methyl, and in anotherparticular embodiment R₂ and R₃ are methyl.

In yet another preferred embodiment, the cetane number improver isdefined by Formula (5):

or Formula (5A):

wherein each of R₁ to R₃, R₅ to R₇, R₉, R_(a) and R_(b) are as definedin connection with any embodiment of Formulas (3) or (3A), or as definedin connection with any embodiment of Formulas (4) or (4A). Preferably,in the compound of Formulas (5) and (5A), R₁, R₆ and R₇ are H; and R₂,R₃, R₅, R₉, R_(a) and R_(b) are independently selected from H, methyland ethyl. Beneficially at least one of R₂, R₃, R₅, R₉, R_(a) and R_(b)is not H, and preferably 1, 2 or 3 of R₂, R₃, R₅, R₉, R_(a) and R_(b) isnot H. Most preferred are compounds wherein three of R₂, R₃, R₅, R₉,R_(a) and R_(b) are methyl.

In the cetane number improver compounds of embodiments of the invention,the nitrate may be arranged in an axial or equatorial position relativeto the alkyl ring. In a preferred embodiment, however, the nitrate isattached in an equatorial position on the ring.

Most preferred cetane improvers of this embodiment are bornyl nitrate,fenchyl nitrate and menthly nitrate.

In accordance with another embodiment of the invention, the cetanenumber improver is a compound of the Formula (6): R_(x)—ONO₂, whereinR_(x) is a linear (straight-chain) aliphatic group having 8 to 24 carbonatoms. In one embodiment, the aliphatic group has between 10 and 20carbon atoms, and preferably between 10 and 18 carbon atoms. Forexample, in one embodiment, R_(x) groups have 8, 10, 12, 13, 14, 16 or18 carbon atoms. In a preferred embodiment, R_(x) group has 18 carbonatoms. The aliphatic group may be saturated or unsaturated. Whenunsaturated it is preferably mono-unsaturated. The R_(x) group mayoptionally be substituted with a nitrate (—ONO₂) group to form adinitrate compound. Preferably the one to two nitrate groups areattached to the terminal carbon atoms of the aliphatic group.

Embodiments of cetane enhancers of the invention of Formula (6) include1-octyl nitrate, 1,10,-decyl dinitrate, 1-dodecyl nitrate, 1-tridecylnitrate, 1-tetradecyl nitrate, 1-hexadecyl nitrate, and 1-octadecylnitrate.

In accordance with one embodiment, the R_(x) group a fatty acidderivative, particularly a fatty alcohol derivative. A preferred fattyalcohol derivative is monounsaturated, and is more preferably acis-monounsaturated fatty alcohol derivative (i.e. wherein adjacenthydrogen atoms are on the same side of the double bond). Thus, in someembodiments, the cetane number improver of the present invention has theFormula (7):

CH₃(CH₂)_(x)CH═CH(CH₂)_(y)ONO₂

wherein X is between 3 and 8 and Y is between 4 and 9. In oneembodiment, X is between 5 and 7 and Y is between 6 and 8. In apreferred embodiment, X is 5 or 7 and Y is 6 or 8. In another preferredembodiment, the carbon-carbon double bond is in the cis formation.

A preferred monounsaturated aliphatic nitrate of the invention is oleylnitrate.

In accordance with a third embodiment of the invention, the cetanenumber improver of the invention is a nitro-substituted fatty acid esterof the Formula (7): R_(y)-[O_(s)NO_(t)]_(u), wherein R_(y) is a alkylester of a fatty acid, i.e. a fatty acid alkyl ester (FAAE) moiety, S is0 or 1, t is 1 or 2, and u is 1 to 4 or 1 to 3.

In one embodiment, the FAAE group is a linear (straight-chain) aliphaticgroup having 8 to 24 carbon atoms, preferably between 10 and 22 carbonatoms, preferably between 12 and 20 carbon atoms, and most preferablybetween 14 and 18 carbon atoms. For example, particularly suitable R_(y)groups have 14, 16 or 18 carbon atoms; and advantageously the R_(y)group has 18 carbon atoms. The aliphatic group may be saturated orunsaturated. Preferably R_(y) is mono-unsaturated, and more preferablythe R_(y) group has a cis double-bond. The R_(y) group may optionally besubstituted with one or two additional nitro-substituted groups offormula O_(s)NO_(t).

A preferred nitro-substituted fatty acid ester of the invention is amethyl oleate substituted with 1, 2 or 3 O_(s)NO_(t) groups.

In accordance with yet another embodiment of the invention, the cetanenumber improver of the invention is a nitro-substituted diterpene of theFormula (8): R_(z)—[O_(s)NO_(t)]_(u), wherein R₁ is a diterpene or analkyl ester of a diterpene, and wherein [O_(s)NO_(t)]_(u) is as definedabove. In one embodiment, the compound of Formula (8) is an alkyl esterof abietic acid. In a preferred embodiment, the ester is a methyl orethyl ester. The R_(z) group may optionally be substituted with one ortwo additional nitro-substituted groups of formula O_(s)NO_(t). Inanother preferred embodiment, the nitro-substituted diterpene isnitro-substituted ethyl abietate substituted with 1, 2 or 3 O_(s)NO_(t)groups.

In accordance with another embodiment of the invention, the cetanenumber improver is a nitro-substituted steroid containing from 1 to 3nitrate groups, such as cholesterol nitrate.

In one embodiment, in use, the cetane number improving additive of theinvention may be pre-dissolved in a suitable solvent, for example an oilsuch as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture; afuel component (which again may be either mineral or Fischer-Tropschderived) compatible with the diesel fuel composition in which theadditive is to be used (for example a middle distillate fuel componentsuch as a gas oil or kerosene); a poly alpha olefin; a so-called biofuelsuch as a fatty acid alkyl ester (FAAB), a Fischer-Tropsch derivedbiomass-to-liquid synthesis product, a hydrogenated vegetable oil, awaste or algae oil or an alcohol such as ethanol; an aromatic solvent;any other hydrocarbon or organic solvent; or a mixture thereof.Preferred solvents for use in this context are mineral oil based dieselfuel components and solvents, and Fischer-Tropsch derived componentssuch as the “XtL” components referred to below. Biofuel solvents mayalso be preferred in certain cases. In one embodiment, the cetaneenhancer will be part of an additive (performance) package additionallycontaining other additives such as detergents, anti-foaming agents,corrosion inhibitors, dehazers etc. Alternatively, the cetane enhancingagent of the invention may be blended directly with the base fuel.

The concentration of the cetane number enhancing additive used maydepend on desirable fuel characteristics/properties, such as: thedesired combustability of the overall fuel composition; thecombustability of the composition prior to incorporation of theadditive; the combustability and/or stability of the additive itself;and/or the properties of any solvent in which the additive is used. Byway of example, the concentration of the cetane number improvingadditive in the fuel composition may be up to 2% w/w and preferably upto 1.0% w/w. Thus, the concentration of the cetane number improver maybe from 0.025% w/w to 2% w/w, or from 0.05% w/w to 1% w/w. In somecases, the concentration of the cetane number improver is from 0.05% w/wto 1.0 w/w, such as 0.05% w/w, 0.1% w/w, 0.25% w/w, 0.5% w/w, 0.75% w/wor 1.0% w/w based on the total weight of the fuel composition.

Where a combination of two or more cetane number improving additives isused in the fuel composition, the same concentration ranges may apply tothe total combination of cetane number improving additives. It will beappreciated that amounts/concentrations may also be expressed as ppm, inwhich case 1% w/w corresponds to 10,000 ppm w/w.

The remainder of the composition will typically consist of one or moreautomotive base fuels optionally together with one or more fueladditives, for instance as described in more detail below.

The relative proportions of the cetane number enhancer, fuel componentsand any other components or additives present in a diesel fuelcomposition prepared according to the invention may also depend on otherdesired properties such as density, emissions performance and viscosity.

The synthesis of embodiments of the cetane enhancers of the inventionare described further below.

Synthesis of Cetane Number Enhancers

A range of alcohols and olefins were evaluated as precursors of cetaneenhancers. The conversion of alcohols to nitrates is generallywell-known in the art (Olah et al. Nitration: Methods and mechanisms,Chapter 4, Aliphatic nitration in Organic nitro chemistry series, VCH,New York, 1989, p. 219; Boschan et al. (1955) Chem. Rev., 55, 485), andany appropriate procedure can be used to produce the organic nitrates ofthe invention.

Although olefinic substrates may afford nitrates (i.e. R—ONO₂) onreaction with typical nitrating agents (HNO₃, N₂O₄, N₂O₅ etc.), moretypically they react to give products with nitro (R—NO₂) and othersubstituents. Since nitroalkanes are inherently unstable (decomposingvia both non-radical, HONO elimination; and radical mechanisms, C—NO₂bond homolysis); like nitrates, they are potential sources of radicalsand may, therefore, also be useful as cetane enhancers.

It was anticipated that, in most cases, the alcohols could be easilyconverted to the corresponding nitrates by reaction with nitric acid.Alcohol nitration involves attack of the —OH functionality on NO₂ ⁺, andmay be generated using the following reagents: (i) nitric acid forsecondary alcohols; (ii) a mixture of nitric acid, sulfuric acid andurea for primary or secondary alcohols; and (iii) a mixture of nitricacid and acetic anhydride (precursor of reactive acetyl nitrate) inacetic acid for unsaturated alcohols.

The choice of reagent used to synthesise the nitrate or nitro-compoundsof the invention may depend on the reactivity of the alcohol startingmaterial and the desirability to avoid side reactions, such as oxidationof the alcohol to a ketone, or sulfation of the olefin. Reactiveolefins, such as pinenes, can be cleanly converted to nitrates, forexample, by (ring-opening) reaction with nitric acid (e.g. Canoira etal. (2007) Fuel, 86, 965; Bakhvalov et al. (2000) J. Organic Chem., 36,1601; Bakhvalov et al. (2002) J. Organic Chem., 38, 507). However,olefins containing ester functionalities (such as unsaturatedconstituents of FAME) may undergo partial hydrolysis of the ester underthese conditions, affording free acid, which is not desirable in adiesel fuel component. In these cases, N₂O₄ may suitably be used insteadof nitric acid. Reaction with olefins normally, therefore, results in amixture of products, containing nitro (R—NO₂), as well as nitrate(R—ONO₂) and hydroxyl functionalities.

Synthesis of embodiments of the cetane enhancing agents of the inventionwere initially performed at small scale (ca. 2 g) to investigate optimumconditions (e.g. yield, purity), and for safety reasons. The reactionswere then scaled up to afford 20 to 40 g of the cetane enhancers, which(following thermal stability tests) were sent for assays in a combustionresearch unit (CRU).

Following the synthesis of the organic nitrates, a differential scanningcalorimetry/thermal gravimetric analysis (DSC/TGA measurement) wasperformed to determine the thermal decomposition behaviour of themolecules, before shipping for further analysis.

Conversion of Primary Alcohols

A common method of converting primary/secondary alcohols to nitratesinvolves the use of mixed nitric and sulfuric acids (molar ratiosubstrate: nitric acid: sulfuric acid generally 1:3:8), together withurea (0.25 equivalent based on substrate). The urea is used to removeany HNO₂ (nitrous acid) formed. The mixed acid is a more powerfulnitrating agent than nitric acid alone, due to the higher concentrationof NO₂ ⁺, which is the reactive species. A general method is shown inScheme 1 below.

The primary alcohols were nitrated at 0° C. for 2 hours. Followingneutralisation, extraction with dichloromethane and drying, the primarynitrates were obtained in a yield of 80-98%. Where a di-nitrate is to besynthesised from a di-hydroxy starting material the relative proportionsof reaction components are selected to achieve two equivalents of HNO₃to ensure complete conversion of hydroxyl groups. In some cases, lowerthan expected yields of nitrated alkyls were obtained, which may be dueto dissolution of part of the nitrated product in the water layer.

Exemplary nitrated alkyls (compounds 1 to 6) of the invention areillustrated in FIG. 1.

Conversion of Secondary Alcohols

In some cases secondary alcohols may be nitrated using the same reagentmixture and conditions as used for primary alcohols. Alternatively,nitric acid may be used, which is a less powerful oxidising agent thanthe mixed acid.

One exemplary secondary alcohol is endo-borneol, which may be convertedinto a bornyl nitrate.

By way of example, the secondary alcohols may be nitrated by reactingwith nitric acid at room temperature (RT) for approximately 24 hours. Inanother example, exo-borneol was reacted with nitric acid at RT toobtain the exo-nitrate of borneol (see reaction Scheme 2 below and FIG.1, compound 7; yield 87%).

Nitrated products from exo-fenchol and endo-fenchol may be obtained in asimilar manner to the borneol compounds (see reaction Scheme 3 below).Unlike endo-fenchol, exo-fenchol is not commercially available, but maybe prepared by the reduction of L-fenchone via aMeerwein-Ponndorf-Verley reduction (MPV; see e.g. Hückel & Rohrer (1960)Chem. Ber., 93, 1053; Mojtahedi et al. (2007) Org. Lett., 9, 2791). Thiswas found to give a mixture of the endo- and exo-fenchol in the ratio1:3. The MPV reaction is an aluminium-catalyzed hydride shift (ofRCHOH), from the alcohol (isopropanol) to the carbonyl carbon(L-fenchone). Isopropanol may be used as a hydride donor, because theacetone formed can be easily removed by distillation. The reaction wasrefluxed at 95° C. for 7 days followed by an extraction. This mixture ofsecondary alcohols was allowed to react with nitric acid for 24 hours atRT, to give a mixture of two nitrated compounds (see FIG. 1, compounds8a, 8b) as the main products, together with unreacted endo-fenchol. Thesterically crowded endo-isomer appears not to react with the mildernitrating reagent, whereas the exo-fenchol affords the two isomericnitrates, as confirmed by ¹H and ¹³C NMR spectroscopy.

The secondary alcohol, menthol (e.g. L-isomer), may be nitrated usingthe same conditions as for primary alcohols in general (i.e. mixed acid;see Scheme 4). After extraction with diethyl ether, the nitrated product(see FIG. 1, compound 9) was obtained in a yield of 95%.

Conversion of α-Terpineol

β-pinene may be reacted with nitric acid, e.g. at −15° C. indichloromethane, according to methods known to the person of skill inthe art (see reaction Scheme 5 below). After neutralisation, the productmay be isolated by extraction with dichloromethane. NMR may be used toconfirm the identity of the product.

As illustrated, this reaction proceeds via protonation of the doublebond in β-pinene, followed by rearrangement (β-fragmentation) to relievering strain and attack of NO₃ ⁻ on the tertiary carbonium ion.

Tertiary nitrates are generally known to be less thermally stable thanprimary and secondary nitrates. Therefore, it may be necessary to storeand ship tertiary nitrates, such as α-terpineol nitrate, with particularcare.

Conversion of Unsaturated Alcohols

In general, unsaturated alcohols can be nitrated selectively at thealcohol position to leave the double bond intact, provided that the modeof introducing the reactants and the amount of nitrating agent issuitably controlled, and the use of sulfuric acid is avoided (as thismay react with the double bond to give a sulfate). Thus, any suitablemethod can be used.

By way of example, oleyl alcohol (e.g. cis-9-octadecen-1-ol) wasnitrated slowly adding nitric acid to a mixture of the alcohol andacetic anhydride in acetic acid solvent at 15° C. (see Scheme 6 below).Advantageously, this embodiment avoids build-up of significantconcentrations of acetyl nitrate (which may undergo runawaydecomposition above 60° C.). Acetyl nitrate is a powerful nitratingagent, being a good source of NO₂ ⁺, which reacts with the alcohol. Theproduct (FIG. 1, compound 10) can then be extracted with a suitablesolvent, such as diethyl ether. 1H-NMR confirmed the clean formation ofthe desired single product without significant by-products.

Nitration of unsaturated secondary alcohols, such as cholesterol, may beperformed similarly so as to form cholesterol nitrate (compound 11)shown in FIG. 1.

Conversion of Olefinic Esters

The formation of nitrate or nitro derivatives of olefins may be carriedout using any appropriate reaction scheme. Typically, conditions usedwill be different to those described above for non-olefinic esters. Forexample, it is desirable to avoid sulfuric acid (to reduce risk ofolefin sulfation). Exemplary conditions include: (i) HNO₃ (70%) andacetic anhydride (precursor of acetyl nitrate); (ii) fuming HNO₃, aceticacid and NaNO₂; (iii) N₂O₄ in chloroform or hexane.

When used in the nitration of FAMEs, processes (i) and (ii) have beenreported to result in the partial hydrolysis of the ester. For example,scheme (i) has been found to form nitro/nitrate and nitro/acetateproducts as well as nitro-substitution of the allylic position. Scheme(ii) has been found to result in low conversion rates of unsaturatedFAMEs, with nitro-olefins formed in low yields. Therefore, reactionscheme (iii) may be preferred, since it is expected to avoid esterhydrolysis. N₂O₄ may conveniently be used as a dilute solution (boilingpoint 30-100° C.), and excess reagent removed under low pressure at theend of the procedure. Chloroform and hexane were both used as solvents.

Three unsaturated FAMEs and a resin ester were nitrated using thisprocedure. In a typical reaction, methyl oleate was added to a stocksolution of N₂O₄ in chloroform at 0 C and stirred for 48 hours. After,excess N₂O₄ was removed in vacuo and the product quenched in an icebath. The products can be extracted in a suitable solvent, such asether, and dried. A mixture of products was identified by ¹H and ¹³C NMRanalysis.

Analysis indicated that the main products of the nitration reaction werethe 1,2-dinitro compound, (O₂N)C(R)H—C(R′)H(NO₂) and nitro alcohols(e.g. NO₂ . . . OH). It is likely that addition of a nitro radical tothe double bond is followed by reaction of the resulting radical with asecond nitro radical to give the 1,2-dinitroalkane and1-nitro-2-nitritoalkane. The nitrite group (R—ONO) in the1-nitro-2-nitrito product tends to undergo hydrolysis during work-up togive the nitro-alcohol. Only small amounts of nitroalkene (RCH═C(R′)NO₂)and the allylic nitro products were observed. The reaction and theresultant mixture of products is illustrated in Scheme 7 below.

Similar reactions were carried out to convert other FAMEs, e.g. methyllinoleate (methyl cis,cis-9,12-octadecadienoate) and methyl linolenate(methyl cis,cis,cis-9,12,15-octadecatrienoate) in chloroform and hexaneas solvents.

Nitration products of abietic acid—the most prevalent of the organicacids that form the largest constituent of rosin—such as ethyl abietatewere also synthesised in similar fashion. Reactions were carried outusing N₂O₄ in either hexane or chloroform and a similar mixture ofnitrogen-containing products was formed. Hexane is a preferred solventfor this reaction. The NMR data was consistent with the formation ofdinitro and nitro-alcohol products containing a single double bond (e.g.1,4-dinitro-2-alkene).

DSC/TGA analysis showed that the product mixture underwent an exothermicdecomposition reaction in the temperature range 150-250° C., making afull analysis of the products difficult.

Since the relative thermal stability of the organic nitrates of theinvention is one of the key characteristics for determine (and explain)how rapidly they combust and, hence, how effective they may be ascombustion improvers (cetane number enhancers) when used in a fuelwithin a diesel engine, thermal stability assays (e.g. differentialscanning calorimetry/thermogravimetric analysis (DSC/TGA)) of theproducts were also conducted, as described in the Examples. Massspectrometry (MS) may also be used to provide information on thedecomposition mechanism.

DSC measures heat flow resulting from evaporation (endotherm) or thermaldecomposition of the compound (exotherm). TGA measures the weight losson evaporation or decomposition. MS of the gas space allows the identityof the volatile decomposition products to be determined. As well as thedecomposition temperature, the associated exotherm may, in principle, bedetermined. These measurements on embodiments of the organic nitratesaccording to the invention were intended to provide an initialassessment of their relative thermal decomposition behaviour and notlimit the scope of the invention.

Diesel Fuel Compositions

In one aspect of the invention, there is provided a diesel fuelcomposition, which comprises an embodiment of a cetane number improverof the invention. In particular, the cetane number improver is presentat a concentration sufficient and appropriate for achieving a desiredcetane number in the resultant fuel composition.

A diesel fuel composition prepared in accordance with aspects of thepresent invention may in general be any type of diesel fuel compositionsuitable for use in a compression ignition (diesel) engine; and it mayitself comprise a mixture of diesel fuel components.

Thus, in addition to the cetane enhancer, a diesel fuel compositionprepared according to aspects of the present invention may comprise oneor more diesel fuel components of conventional type. It may, forexample, include a major proportion of a diesel base fuel, for instanceof the type described below. In this context, a “major proportion” meansat least 50% w/w, and typically at least 85% w/w based on the overallcomposition. In a preferred embodiment, a “major proportion” alsoincludes at least 90% w/w or at least 95% w/w, and in some cases, atleast 98% w/w or at least 99% w/w of the fuel composition consists ofthe diesel base fuel. Accordingly, in some embodiments, the base fuelmay itself comprise a mixture of two or more diesel fuel components ofthe types described below.

Typical diesel fuel components comprise liquid hydrocarbon middledistillate fuel oils, for instance petroleum derived gas oils. Such basefuel components may be organically or synthetically derived, and areobtained by distillation of a desired range of fractions from a crudeoil. They will typically have boiling points within the usual dieselrange of 150 to 410° C. or 170 to 370° C., depending on grade and use.They will typically have densities from 0.75 to 0.9 g/cm³, such as from0.8 to 0.86 g/cm³, at 15° C. (IP 365) and measured cetane numbers (ASTMD613) of from 35 to 80, more preferably from 40 to 75. Their initialboiling points will be in the range 150 to 230° C. and their finalboiling points in the range 290 to 400° C. Their kinematic viscosity at40° C. (ASTM D445) might suitably be from 1.5 to 4.5 centistokes. Suchfuels are generally suitable for use in compression ignition (diesel)internal combustion engines, of either the indirect or direct injectiontype.

An automotive diesel fuel composition which results from carrying outaspects of the present invention also falls within these generalspecifications or standards. Accordingly, it will generally comply withapplicable current standard specification(s) such as for example EN 590(for Europe) or ASTM D975 (for the USA). By way of example, the fuelcomposition may have a density from 0.82 to 0.845 g/cm³ at 15° C.; a T₉₅boiling point (ASTM D86) of 360° C. or less; a cetane number (ASTM D613)of 45 or greater; a kinematic viscosity (ASTM D445) from 2 to 4.5 mm²/sat 40° C.; a sulphur content (ASTM D2622) of 50 mg/kg or less; and/or apolycyclic aromatic hydrocarbons (PAH) content (IP391 (mod)) of lessthan 11% w/w. Relevant specifications may, however, differ from countryto country and from year to year and may depend on the intended use ofthe fuel composition. In particular, its measured cetane number willpreferably be from 45 to 70, to 75 or to 80, more preferably from 50 to65, or at least greater than 50, greater than 55, greater than 60, orgreater than 65.

A petroleum derived gas oil, e.g., obtained from refining and optionally(hydro)processing a crude petroleum source, may be incorporated into adiesel fuel composition. It may be a single gas oil stream obtained fromsuch a refinery process or a blend of several gas oil fractions obtainedin the refinery process via different processing routes. Examples ofsuch gas oil fractions are straight run gas oil, vacuum gas oil, gas oilas obtained in a thermal cracking process, light and heavy cycle oils asobtained in a fluid catalytic cracking unit, and gas oil as obtainedfrom a hydrocracker unit. Optionally, a petroleum derived gas oil maycomprise some petroleum derived kerosene fraction. Such gas oils may beprocessed in a hydro-desulphurisation (HDS) unit so as to reduce theirsulphur content to a level suitable for inclusion in a diesel fuelcomposition. This also tends to reduce the content of other polarspecies such as oxygen- or nitrogen-containing species. In some cases,the fuel composition will include one or more cracked products obtainedby splitting heavy hydrocarbons.

In some embodiments of the present invention, the base fuel may be orcontain another so-called “biodiesel” fuel component, such as avegetable oil, hydrogenated vegetable oil or vegetable oil derivative(e.g. a fatty acid ester (FAE), in particular a fatty acid methyl ester(FAME)), or another oxygenate such as an acid, ketone or ester. Suchcomponents need not necessarily be bio-derived. Where the fuelcomposition contains a biodiesel component, the biodiesel component maybe present in quantities up to 100%, such as between 1% and 99% w/w,between 2% and 80% w/w, between 2% and 50% w/w, between 3% and 40% w/w,between 4% and 30% w/w, or between 5% and 20% w/w. In one embodiment,the biodiesel component may be FAME.

A diesel base fuel may consist of or comprise a Fischer-Tropsch deriveddiesel fuel component, typically a Fischer-Tropsch derived gas oil. Asused herein, the term “Fischer-Tropsch derived” means that a materialis, or is obtained from, a synthesis product of a Fischer-Tropschcondensation process. A Fischer-Tropsch derived fuel or fuel componentwill therefore be a hydrocarbon stream in which a substantial portion,except for added hydrogen, is derived directly or indirectly from aFischer-Tropsch condensation process.

Fischer-Tropsch fuels may be derived by converting gas, biomass or coalto liquid (XtL), specifically by gas to liquid conversion (GtL), or frombiomass to liquid conversion (BtL). Any form of Fischer-Tropsch derivedfuel component may be used as a base fuel in accordance with aspects ofthe invention.

In one embodiment, the base fuel has a low sulphur content, for exampleat most 1000 mg/kg (1000 parts per million by weight/ppmw). In apreferred embodiment, it will have a low or ultra low sulphur content,for instance at most 500 mg/kg (500 ppmw), such as no more than 350mg/kg (350 ppmw), and still more preferably no more than 100 or 50 or 10or even 5 mg/kg (5 ppmw) of sulphur. It may be a so-called“zero-sulphur” fuel; although in some cases it may be desired that thebase fuel is not a sulphur free (“zero sulphur”) fuel. In a preferredembodiment, a fuel composition which results from carrying out aspectsof the present invention will also have a sulphur content falling withinthese limits.

The diesel fuel composition according to aspects of the presentinvention may, if desired, contain no, or only low levels of additionalcetane improving (ignition improving) additives such as 2-ethylhexylnitrate (2-EHN). In other words, embodiments of the present inventionembrace the use of certain organic nitrates in a diesel fuel compositionfor the purpose of reducing the level of a second (or further) cetaneimproving additive in the composition.

Furthermore, a fuel composition prepared according to aspects of thepresent invention, or a base fuel used in such a composition may containone or more fuel additives, or may be additive-free. If additives areincluded (e.g. added to the fuel at the refinery), the composition maycontain minor amounts of one or more additives. Selected examples orsuitable additives include (but are not limited to): anti-static agents;pipeline drag reducers; flow improvers (e.g. ethylene/vinyl acetatecopolymers or acrylate/maleic anhydride copolymers); lubricity enhancingadditives (e.g. ester- and acid-based additives); viscosity improvingadditives or viscosity modifiers (e.g. styrene-based copolymers,zeolites, and high viscosity fuel or oil derivatives); dehazers (e.g.alkoxylated phenol formaldehyde polymers); anti-foaming agents (e.g.polyether-modified polysiloxanes); anti-rust agents (e.g. apropane-1,2-diol semi-ester of tetrapropenyl succinic acid, orpolyhydric alcohol esters of a succinic acid derivative); corrosioninhibitors; reodorants; anti-wear additives; antioxidants (e.g.phenolics such as 2,6-di-tert-butylphenol); metal deactivators;combustion improvers; static dissipator additives; cold flow improvers(e.g. glycerol monooleate, di-isodecyl adipate); antioxidants; and waxanti-settling agents. The composition may for example contain adetergent. Detergent-containing diesel fuel additives are known andcommercially available. Such additives may be added to diesel fuels atlevels intended to reduce, remove or slow the build up of enginedeposits. In some embodiments, it may be advantageous for the fuelcomposition to contain an anti-foaming agent, more preferably incombination with an anti-rust agent and/or a corrosion inhibitor and/ora lubricity enhancing additive.

Where the composition contains such additives (other than the cetanenumber increasing components of the invention), it preferably contains aminor proportion (such as 1% w/w or less, 0.5% w/w or less, 0.2% w/w orless), of the one or more fuel additives, in addition to the cetanenumber increasing component(s). Unless otherwise stated, the (activematter) concentration of each such additive component in the fuelcomposition may be up to 10000 ppmw, such as in the range of 0.1 to 1000ppmw; and advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150ppmw.

If desired, one or more additive components, such as those listed above,may be co-mixed (e.g. together with suitable diluent) in an additiveconcentrate, and the additive concentrate may then be dispersed into abase fuel or fuel composition. In some cases, it may be possible andconvenient to incorporate the cetane number increasing component of theinvention into such an additive formulation. Thus, the cetane numberimproving additive may be pre-diluted in one or more such fuelcomponents, prior to its incorporation into the final automotive fuelcomposition. Such a fuel additive mixture may typically contains adetergent, optionally together with other components as described above,and a diesel fuel-compatible diluent, which may be a mineral oil, asolvent such as those sold by Shell companies under the trade mark“SHELLSOL”, a polar solvent such as an ester and, in particular, analcohol (e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol andalcohol mixtures such as those sold by Shell companies under the trademark “LINEVOL”, especially LINEVOL 79 alcohol which is a mixture of C₇₋₉primary alcohols, or a C₁₂₋₁₄ alcohol mixture which is commerciallyavailable).

In one embodiment, the total content of the additives in the fuelcomposition may be between 0 and 10000 ppmw and preferably below 5000ppmw.

As used herein, amounts (e.g. concentrations, ppmw and % w/w) ofcomponents are of active matter, i.e., exclusive of volatilesolvents/diluent materials.

In one embodiment, the present invention involves adjusting the cetanenumber of the fuel composition, using the cetane number enhancingcomponent, in order to achieve a desired target cetane number.

The maximum cetane number of an automotive fuel composition may often belimited by relevant legal and/or commercial specifications, such as theEuropean diesel fuel specification EN 590 that stipulates a cetanenumber of 51. Thus, typical commercial automotive diesel fuels for usein Europe are currently manufactured to have cetane numbers of around51. Thus, embodiments of the present invention may involve manipulationof an otherwise standard specification diesel fuel composition, using acetane number enhancing additive, to increase its cetane number so as toimprove the combustability of the fuel, and hence reduce engineemissions and even fuel economy of an engine into which it is, or isintended to be, introduced.

In one embodiment, the cetane number improver increases the cetanenumber of the fuel composition by at least 3 cetane numbers. In someparticular embodiments, the cetane number increase may be up toapproximately 9, or any value in between these ranges. Accordingly, inother embodiments, the cetane number of the resultant fuel is between 51and 60.

In a preferred embodiment, an automotive diesel fuel compositionprepared according to aspects of the present invention will comply withapplicable current standard specification(s) such as, for example, EN590 (for Europe) or ASTM D-975 (for the USA). By way of example, theoverall fuel composition may have a density from 820 to 845 kg/m³ at 15°C. (ASTM D-4052 or EN ISO 3675); a T95 boiling point (ASTM D-86 or ENISO 3405) of 360° C. or less; a measured cetane number (ASTM D-613) of51 or greater; a VK 40 (ASTM D-445 or EN ISO 3104) from 2 to 4.5 mm²/s;a sulphur content (ASTM D-2622 or EN ISO 20846) of 50 mg/kg or less;and/or a polycyclic aromatic hydrocarbons (PAH) content (IP 391 (mod))of less than 11% w/w. Relevant specifications may, however, differ fromcountry to country and from year to year, and may depend on the intendeduse of the fuel composition.

It will be appreciated, however, that diesel fuel composition preparedaccording to aspects of the present invention may contain fuelcomponents with properties outside of these ranges, since the propertiesof an overall blend may differ, often significantly, from those of itsindividual constituents.

Uses and Methods

In accordance with one aspect of the invention, there is provided theuse of an embodiment of the cetane number improver of the invention toachieve a desired cetane number of the resultant fuel composition. Insome embodiments, the desired cetane number is achieved or intended tobe achieved under a specified set or range of engine working conditions,as described elsewhere herein. Accordingly, an advantage of embodimentsof the present invention is that cetane number enhancers of theinvention may be suitable for reducing the combustion delay of a fuelcomposition under all engine running conditions, or under mild, or underharsh engine conditions. Embodiments of the cetane number enhancer ofthe invention may serve to improve combustion and, hence, improveassociated engine factors, such as exhaust emissions and/or enginedeposits under a range of engine operating conditions—particularly underharsh engine conditions when fuel emissions might otherwise be expectedto increase.

In the context of the present invention, “use” of a cetane numberimprover in a fuel composition means incorporating the component intothe composition, typically as a blend (i.e. a physical mixture) with oneor more fuel components (typically diesel base fuels) and optionallywith one or more fuel additives.

The cetane number improver is preferably incorporated into the fuelcomposition before the composition is introduced into an engine which isto be run on the composition. Accordingly, the viscosity increasingcomponent may be dosed directly into (e.g. blended with) one or morecomponents of the fuel composition or the base fuel at the refinery. Forinstance, it may be pre-diluted in a suitable fuel component, whichsubsequently forms part of the overall automotive fuel composition.Alternatively, it may be added to a diesel fuel composition downstreamof the refinery. For example, it may be added as part of an additivepackage containing one or more other fuel additives. This can beparticularly advantageous because in some circumstances it can beinconvenient or undesirable to modify the fuel composition at therefinery. For example, the blending of base fuel components may not befeasible at all locations, whereas the introduction of fuel additives,at relatively low concentrations, can more readily be achieved at fueldepots or at other filling points such as road tanker, barge or trainfilling points, dispensers, customer tanks and vehicles.

Accordingly, the “use” of embodiments of the invention may alsoencompass the supply of a cetane number improver together withinstructions for its use in a diesel fuel composition to achieve one ofthe benefits of the present invention. The cetane number increasingcomponent may therefore be supplied as a component of a formulationwhich is suitable for and/or intended for use as a fuel additive, inparticular a diesel fuel additive. By way of example, the cetane numberimprover may be incorporated into an additive formulation or packagealong with one or more other fuel additives. As described above, the oneor more fuel additives may be selected from any useful additive, such asdetergents, anti-corrosion additives, esters, poly-alpha olefins, longchain organic acids, components containing amine or amide activecentres, and any combination thereof, as is known to the person of skillin the art.

According to another aspect of the invention, there is provided aprocess for the preparation of an automotive fuel composition, whichprocess involves blending a diesel base fuel (or base fuel mixture) withan embodiment of the cetane number improver of the invention. Theblending may be carried out for one or more of the purposes describedherein.

In some cases the cetane number improver of the invention may not besuitable for pre-mixing with other fuel additives and may, therefore, bedosed directly into the fuel composition from a concentrated (100%) orpre-diluted stock.

In accordance with one embodiment of the present invention, two or morecetane number increasing additives may be used in a diesel fuelcomposition to provide one or more of the effects of the inventiondescribed herein.

For example, embodiments of the present invention can provide aneffective way of improving fuel combustion/combustability in an internalcombustion engine.

It has surprisingly been found that certain organic nitrate molecules ofthe invention can, at relatively low concentrations, increase the cetanenumber of a diesel fuel composition by an amount greater than knownorganic nitrate cetane enhancers under some engine operating conditions.In particular, embodiments of the cetance number enhancing agents of theinvention may be capable of providing greater benefits than some priorart cetane number improvers, particularly under harsh engine workingconditions (e.g. high engine speeds and powers).

While the amount of the cetane number increasing component for use inaccordance with aspects of the invention may vary depending of fuel typeand/or engine working conditions to be used; a further benefit of theinvention is that under some engine conditions the amount of cetanenumber improver needed to observe the benefit of the invention may besurprisingly low, such as at the level of typical fuel additives.

This in turn can reduce the cost and complexity of the fuel preparationprocess. For example, it can allow a fuel composition to be altered inorder to improve fuel combustability, by the incorporation of additivesdownstream of the refinery, rather than by altering the content of thebase fuel at its point of initial preparation. The blending of base fuelcomponents may not be feasible at all locations, whereas theintroduction of fuel additives, at relatively low concentrations, canmore readily be achieved at fuel depots or at other filling points suchas road tanker, barge or train filling points, dispensers, customertanks and vehicles. This in particular may be achievable where thecetane number improver is sufficiently stable to allow it to betransported under suitable conditions without taking unnecessary safetyrisks. Of course, in some case it may not be appropriate due to safetyfactors to transport the cetane number improver.

Moreover, an additive which is to be used at a relatively lowconcentration can naturally be transported, stored and introduced into afuel composition more cost effectively than can a fuel component whichneeds to be used at concentrations of the order of tens of percent byweight.

Another aspect of the invention provides a method of operating aninternal combustion engine and/or a vehicle powered by such an engine,which comprises introducing into a combustion chamber of the engine afuel composition prepared in accordance with aspects of the invention.The fuel composition is advantageously introduced for one or more of thepurposes described in connection with aspects of this invention. Thus,the engine is preferably operated with the fuel composition for thepurpose of improving ease of fuel ignition during use of the engine (byincreasing fuel combustability) and, for example, associated benefitssuch as reduced engine emissions, engine noise, etc. The engine is inparticular a diesel engine, and may be a turbo charged diesel engine.The diesel engine may be of the direct injection type, for example ofthe rotary pump, in-line pump, unit pump, electronic unit injector orcommon rail type, or of the indirect injection type. It may be a heavyor a light duty diesel engine. For example, it may be an electronic unitdirect injection (EUDI) engine.

Where relevant to a particular assessment, emission levels may bemeasured using standard testing procedures such as the European R49,ESC, OICA or ETC (for heavy-duty engines) or ECE+EUDC or MVEG (forlight-duty engines) test cycles. In a preferred embodiment, emissionsperformance is measured on a diesel engine built to comply with the EuroII standard emissions limits (1996) or with the Euro III (2000), IV(2005) or even V (2008) standard limits.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Thus features, integers, characteristics, compounds, chemical moietiesor groups described in conjunction with a particular aspect, embodimentor example of the present invention are to be understood to beapplicable to any other aspect, embodiment or example described hereinunless incompatible therewith. Thus, features of the “uses” of theinvention are directly applicable to the “methods” of the invention.Moreover, unless stated otherwise, any feature disclosed herein may bereplaced by an alternative feature serving the same or a similarpurpose.

Aspects of the invention will now be further illustrated by way of thefollowing non-limiting examples.

EXAMPLES Introduction

Various organic nitrates were synthesised and assayed for theirpotential to act as cetane enhancers in diesel fuels. Cetane enhancersof the invention may provide various benefits, in use, that areassociated with increased combustability and a reduction in ignitiondelay, such as reducing engine noise, reducing build-up of enginedeposits, reducing engine emissions, and may even improve fuel economyin some cases.

Organic nitrates of the invention were also tested for thermal stabilityto obtain useful information on necessary storage and transportationconditions.

Organic nitrate synthesis processes may also help in determiningsuitability of the organic nitrates for use as cetane enhancers indiesel fuel at a commercial level (e.g. with respect to ease ofsynthesis and cost of production).

Reagents and Chemicals

In general, standard reagents, chemicals and solvents were purchasedfrom Sigma-Aldrich. Ethyl abietate (70%) was obtained from TCI and ABCR.Methyl linoleate was purchased from TCI. Neodol 23 (C12-C13 alcoholmixture) was obtained from Shell Chemicals.

To form the solution of N₂O₄, gaseous yellow/brown NO₂/N₂O₄(Sigma-Aldrich) was slowly bubbled into the solvent (chloroform orhexane) in a round-bottomed-flask in an ice bath. The dew point of theN₂O₄ is around 20° C., so the heavy gas condenses into the cold solvent(at ca. 4° C.). The amount of N₂O₄ added was determined by weighing theflask before and after addition of N₂O₄.

General

The following general synthesis methods were used for the preparation ofnitrate and nitro compounds of the invention:

Nitric acid, for secondary alcohols

Mixture of nitric acid, sulfuric acid and urea, for primary or secondaryalcohols

Mixture of nitric acid and acetic anhydride (precursor of reactiveacetyl nitrate) in acetic acid, for unsaturated alcohols

N₂O₄, for olefins

A 500 ml three-necked round-bottomed flask, equipped with a ‘bubbler’,thermometer, a dropping funnel and a magnetic stirring bar was used inall the experiments. Reactions were performed under nitrogen.

Synthesis of Organic Nitrates Nitration Product of 1-Octanol

A mixture of concentrated nitric acid (45 ml, 0.96 mol), concentratedsulfuric acid (134 ml, 2.35 mol) and urea (5 g, 0.08 mol) were placed ina 500 m three-necked flask and cooled to 0° C. 1-Octanol (48 ml, 0.30mol) in dichloromethane (30 ml) was added from a dropping funnel for 60min. The temperature was maintained at 0 to 5° C. with an ice-salt bath.After 2 hours the reaction mixture was poured in an ice bath (quenched)and dichloromethane (100 ml) was added. The organic phase was washed forthree times with water, followed by sodium bicarbonate. The organicphase was dried over magnesium sulfate and the solvent was evaporated bya rotorvap (40° C. bath temperature, 5 mbar vacuum). The yield was 43 g(81%).

¹H NMR (CDCl₃, 300 MHz): 4.4 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.1 (m, 10H),0.9-0.8 (m, 3H)

¹³C NMR (CDCl₃): 73.7, 32.0, 29.5, 29.4, 27.1, 26.0, 23.0, 14.4

Nitration Product of 1,10-Decanediol

A mixture of concentrated nitric acid (67 ml, 1.44 mol), concentratedsulfuric acid (200 ml, 3.50 mol) and urea (7 g, 0.12 mol) were placed ina 500 ml three-necked flask and cooled to 0° C. 1,10-octanediol (48 ml,0.30 mol) in dichloromethane (30 ml) was added from a dropping funnelfor 60 min. The temperature was maintained at 0 to 5° C. with anice-salt bath. After 2 hours the reaction mixture was poured in an icebath (quenched) and dichloromethane (100 ml) was added. The organicphase was washed for three times with water, followed by sodiumbicarbonate. The organic phase was dried over magnesium sulfate and thesolvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbarvacuum). The yield was 49 g (98%).

¹H NMR (CDCl₃, 300 MHz): 4.4 (t, 4H), 1.8-1.6 (m, 4H), 1.4-1.2 (m, 12H)

¹³C NMR (CDCl₃): 73.7, 29.5, 29.3, 27.0, 25.9

Nitration Product of 1-Tetradecanol

A mixture of concentrated nitric acid (27 ml, 0.58 mol), concentratedsulfuric acid (81 ml, 1.42 mol) and urea (3 g, 0.05 mol) were placed ina 500 ml three-necked flask and cooled to 0° C. 1-Tetradecanol (48 ml,0.19 mol) in dichloromethane (30 ml) was added from a dropping funnelfor 45 min. The temperature was maintained at 0 to 5° C. with anice-salt bath After 2 hours the reaction mixture was poured in an icebath (quenched) and dichloromethane (100 ml) was added. The organicphase was washed for three times with water, followed by sodiumbicarbonate. The organic phase was dried over magnesium sulfate and thesolvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbarvacuum). The yield was 38 g (79%).

¹H NMR (CDCl₃, 300 MHz): 4.4 (t, 2H), 1.8-1.6 (m, 2H), 1.5-1.2 (m, 22H),0.9-0.8 (m, 3H)

¹³C NMR (CDCl₃): 73.7, 32.3, 30.0, 29.9, 29.8, 29.7, 29.5, 26.0, 23.1,14.5

Nitration Product of Hexadecyl Alcohol

A mixture of concentrated nitric acid (24 ml, 0.52 mol), concentratedsulfuric acid (72 ml, 1.26 mol) and urea (2 g, 0.04 mol) were placed ina 500 ml three-necked flask and cooled to 0° C. Hexadecyl alcohol (40 g,0.16 mol) in dichloromethane (30 ml) was added from a dropping funnelfor 60 min. The temperature was maintained at 0 to 5° C. with anice-salt bath. After 2 hours the reaction mixture was poured in an icebath (quenched) and dichloromethane (100 ml) was added. The organicphase was washed for three times with water, followed by sodiumbicarbonate. The organic phase was dried over magnesium sulfate and thesolvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbarvacuum). The yield was 39 g (80%).

¹H NMR (CDCl₃, 300 MHz): 4.5 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.2 (m, 26H),0.9-0.8 (m, 3H)

¹³C NMR (CDCl₃): 73.6, 32.3, 30.1, 29.8, 27.0, 26.0, 23.1, 14.5

Nitration Product of 1-Octadecanol

A mixture of concentrated nitric acid (16 ml, 0.35 mol), concentratedsulfuric acid (48 ml, 0.85 mol) and urea (2 g, 0.03 mol) were placed ina 500 ml three-necked flask and cooled to 0° C. 1-Octadecanol (36 g,0.11 mol) in dichloromethane (30 ml) was added from a dropping funnelfor 60 min. The temperature was maintained at 0 to 5° C. with anice-salt bath. After 2 hours the reaction mixture was poured in an icebath (quenched) and dichloromethane (100 ml) was added. The organicphase was washed for three times with water, followed by sodiumbicarbonate. The organic phase was dried over magnesium sulfate and thesolvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbarvacuum). The yield was 28 g (80%).

¹H NMR (CDCl₃, 300 MHz): 4.5 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.2 (m, 30H),0.9-0.8 (m, 3H)

¹³C NMR (CDCl₃): 73.7, 32.3, 30.1, 30.0, 29.9, 29.7, 29.5, 27.1, 26.0,23.1, 14.5

Nitration Product of Neodol 23

A mixture of concentrated nitric acid (31 ml, 0.66 mol), concentratedsulfuric acid (92 ml, 1.61 mol) and urea (3 g, 0.05 mol) were placed ina 500 ml three-necked flask and cooled to 0° C. Neodol 23 (48 ml, 0.21mol) in dichloromethane (30 ml) was added from a dropping funnel for 60min. The temperature was maintained at 0 to 5° C. with an ice-salt bath.After 2 hours the reaction mixture was poured in an ice bath (quenched)and dichloromethane (100 ml) was added. The organic phase was washed forthree times with water, followed by sodium bicarbonate. The organicphase was dried over magnesium sulfate and the solvent was evaporated bya rotorvap (40° C. bath temperature, 5 mbar vacuum). A mixture ofdodecyl and tridecyl nitrates was obtained with a yield of 45 g (93%).

¹H NMR (CDCl₃, 300 MHz): 4.5 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.2 (m, 16H),0.9-0.8 (m, 3H)

¹³C NMR (CDCl₃): 73.7, 32.3, 30.0, 29.9, 29.7, 29.5, 27.1, 26.0, 23.1,14.5

Nitration Product of Exo-Borneol

In a 500 ml three neck-flask filled with nitric acid (150.21 ml, 3.24mol) was added exo-borneol (50 g, 0.32 mol) slowly (3.5 hours) at roomtemperature. After 2 hours the reaction mixture was poured in an icebath (quenched) and diethyl ether (200 ml) was added. The organic phasewas washed for three times with water, followed by sodium bicarbonate.The organic phase was dried over magnesium sulfate and was the solventwas evaporated by a rotorvap, 40° C. bath temperature and ±5 mbarvacuum. The yield was 57 g (87%).

¹H NMR (CDCl₃, 300 MHz): 4.8 (t, 1H), 2.0-1.9 (m, 2H), 1.8-1.6 (m, 3H),1.3-1.1 (m, 2H), 1.0-0.9 (m, 6H), 0.8 (s, 3H)

¹³C NMR (CDCl₃): 90.5, 49.9, 47.4, 45.1 38.3, 34.5, 27.2, 20.2, 11.5

Nitration Product of Exo-Fenchol

In a 500 ml three neck-flask was filled L-Fenchone (40 g, 0.26 mol),aluminium isopropoxide (2 g) and isopropyl alcohol (300 ml). Thismixture was refluxed (during reflux the acetone was evaporated andfilled with isopropyl alcohol) for 168 hours, extracted with diethylether, dried and evaporated. 25 g exolendo-fenchol in ratio 3:1 wasisolated.

In a 500 ml three neck-flask filled with nitric acid (75.10 ml, 1.62mol) was added the exo/endofenchol mixture (25 g, 0.16 mol) slowly (2hours) at room temperature. After 2 hours the reaction mixture waspoured in an ice bath (quenched) and diethyl ether (100 ml) was added.The organic phase was washed for three times with water, followed bysodium bicarbonate. The organic phase was dried over magnesium sulfateand the solvent was evaporated using a rotorvap, 40° C. bath temperatureand ±5 mbar vacuum. The yield was 26 g (82%).

¹H NMR (CDCl₃, 300 MHz): 4.8-4.6 (m, 1H), 4.2-4.2 (m, 1H), ratio 1:0.47

¹³C NMR (CDCl₃): 97.2, 88.2

Nitration Product of L-Menthol

A mixture of concentrated nitric acid (15.79 ml, 0.34 mol), concentratedsulphuric acid (47.37 ml, 0.83 mol) and urea (1.62 g, 0.03 mol) wereplaced in a 500 ml three necked flask and cooled to 0° C. L-menthol (17g, 0.11 mol) in diethyl ether (20 ml) was added from a dropping funnelfor 60 min. The temperature was maintained at 0 to 5° C. with anice-salt bath. After 2 hours the reaction mixture was poured in an icebath (quenched) and diethyl ether (100 ml) was added. The organic phasewas washed for three times with water, followed by sodium bicarbonate.The organic phase was dried over magnesium sulfate and was the solventwas evaporated by a rotorvap, 40° C. bath temperature and ±5 mbarvacuum. The yield was 41 g (95%).

¹H NMR (CDCl₃, 300 MHz): 4.9-4.8 (m, 1H), 2.2-1.9 (m, 2H), 1.8-1.7 (m,2H), 1.6-1.4 (m, 2H), 1.2-1.0

(m, 2H), 1.0-0.9 (m, 6H), 0.9-0.8 (m, 3H)

¹³C NMR (CDCl₃): 84.4, 45.8, 39.7, 34.2, 31.8, 26.5, 24.2, 22.2, 20.8,16.8

Nitration Product of Oleyl Alcohol

To a mixture of acetic anhydride (80 mL), acetic acid (80 ml) and oleylalcohol (40 g) at 15° C., was added nitric acid (10 ml) drop wise (60min). After 30 min the reaction mixture was poured in an ice bath(quenched) and diethyl ether (100 ml) was added. The organic phase waswashed for three times with water, followed by sodium bicarbonate. Theorganic phase was dried over magnesium sulfate and was the solvent wasevaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum.The yield was 34 g (74%).

¹H NMR (CDCl₃, 300 MHz): 5.4-5.2 (m, 2H), 4.4 (t, 2H), 2.1-1.9 (m, 4H),1.4-1.2 (24H), 1.0-0.8 (m, 3H)

¹³C NMR (CDCl₃): 130.2, 129.8, 73.7, 32.3, 30.1, 30.0, 29.9, 29.7, 29.6,29.5, 29.4, 27.6, 27.5, 27.1, 26.0, 23.1, 14.5

Nitration Product of Cholesterol

To a mixture of cholesterol (35 g) in chloroform (10 ml) and aceticanhydride (90 ml) in chloroform (10 ml) at 15° C., was added drop wise(60 min) a mixture of nitric acid (12.6 ml) in acetic acid (45 ml).After 60 min the reaction mixture was poured in an ice bath (quenched)and diethyl ether (100 ml) was added. The organic phase was washed forthree times with water, followed by sodium bicarbonate. The organicphase was dried over magnesium sulfate and was the solvent wasevaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum.The yield was 24 g (63%). Cholesterol nitrate is a solid at RT.

¹H NMR (CDCl₃, 300 MHz): 5.5-5.4 (m, 1H), 4.9-4.7 (m, 1H), 2.6-2.3 (m,2H), 2.1-1.1 (m, 25H), 1.1-0.9 (m, 15H), 0.8-0.6 (m, 3H)

¹³C NMR (CDCl₃): 138.3, 122.3, 83.5, 57.6, 56.5, 50.2, 42.5, 40.2, 40.1,37.8, 37.5, 31.8, 29.5, 26.8, 23.4, 18.5, 17.5, 11.2

Nitration Product of Methyl Oleate

To a solution of N₂O₄ (13.96 g, 0.15 mol) in chloroform (20 ml) at 0° C.was added drop wise (3 hours) methyl oleate (30 g, 34.48 ml). Thereaction mixture was stirred for 48 hours with a low nitrogen flow. Thereaction mixture was poured in an ice bath (quenched) and diethyl ether(50 ml) was added. The organic phase was washed for three times withwater. The organic phase was dried over magnesium sulfate and was thesolvent was evaporated by a rotorvap, 40° C. bath temperature and ±5mbar vacuum. The yield was 31.2 g.

Nitration Product of Ethyl Abietate

To a solution of N₂O₄ (12.75 g, 0.14 mol) in hexane (20 ml) at 0° C. wasadded drop wise (2 hours) ethyl abietate (30 g, 34.48 ml). The reactionmixture was stirred for 48 hours with a low nitrogen flow. The reactionmixture was poured in an ice bath (quenched) and diethyl ether (50 ml)was added. The organic phase was washed for three times with water. Theorganic phase was dried over magnesium sulfate and was the solvent wasevaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum.The yield was 33.2 g. The products of the reaction are solid at RT.

Analytical Methods

¹H and ¹³C NMR spectra were recorded on a Varian Mercury 300 MHz or aVarian Inova 400 MHz system. All NMR samples were measured in CDCl₃.

Infrared spectra were measured on a Nicolet 6700 FT-IR Spectrometer(Fourier transform infrared spectroscopy) from Thermo Scientific.

Gas chromatography-mass spectrometry (GC-MS) analyses were performed ona Trace GC Ultra chromatograph from Interscience equipped with a 50m×0.2 mm×0.5 μm RTX-1 PONA column and an DSQII mass-selective EIdetector. The following temperature profile was used in the GC-MS formeasuring the components. The oven started at 35° C. for the first 5min, then increased with 10° C./min to 300° C. followed by a hold timeof 10 min at 300° C. DSC/TGA was measured on a STA 409 PC (SimultaneousThermal Analysis) with QMS 403 C (quadrupole mass spectrometers) fromNETZSCH.

Testing of Blended Fuels

The organic nitrate (or nitro) cetane number enhancing agents wereperformance tested in diesel fuel blends. The assessment of theirability to increase the cetane number of the fuel was carried outindirectly by measuring changes in ignition delay (ID) of the blendedfuels using a combustion research unit (CRU). The CRU is operated todetermine ignition quality in a similar fashion to the ignition qualitytester (IQT; International Standard EN 15195:2007:E) known to the personof skill in the art, i.e. using a heated, pressurised constant volumechamber. Typically, the cetane number is given as a dimensionlessnumber, which describes the ignition behaviour of a fuel in comparisonto primary reference fuels (PRFs) with a defined cetane number. PRFs arebimodal mixtures of n-hexadecane (or “cetane”; CN=100) andheptamethylnonane (CN=15).

Fuel Blending

All test compounds were blended into an EN590 compliant diesel (Europeanspecification), zero-sulfur fuel (i.e. ZSD base-fuel from Stanlowrefinery; density 837.4 kg/m³ at 15° C.; viscosity 2.89 mm²/s at 40° C.)at concentrations of 0.1% w/w and 1.0% w/w. Some test compounds werealso blended at concentrations of 0.05% w/w and 0.5% w/w. The mixtureswere stirred at RT for 1 hour, after which all solid compounds weredissolved in the base fuel.

For purposes of comparison, similar fuel blends containing 2-EHN wereprepared.

Combustion Research Unit (CRU) Testing

In CRU tests, diesel-like fuel is injected into a high temperature, highpressure chamber where it mixes with the hot air and ignites, thusmimicking combustion in a compression-ignition engine. The combustionprocess is monitored via a pressure sensor inside the chamber.

The CRU delivers p-t-charts of the ignition process from which theignition delay, the burn rate and the maximum pressure increase (MPI)can be determined. A comparison of blended fuels with standard fuelsdemonstrates changes in ignition delay/combustability of the fuels, andmay also allow determination of cetane numbers under different operatingconditions.

The engine conditions selected for CRU tests are designed to simulate awide range of engine operating conditions so as to assess the cetaneenhancers under mild, intermediate and harsh conditions; e.g.temperature and pressure are both varied from low to high. This allowsthe temperature and pressure dependence of ignition delay to bedemonstrated. Thus, the slope of the isothermal or isobaric chartsplotted from the data obtained from these tests provide directinformation about how the response of the fuel blend changes from mildto harsh conditions.

Each of the fuel blends (at each concentration of cetane enhancer used)and the base fuels were tested for ignition quality on the CRU under the11 different sets of parameters (engine operating conditions)illustrated in Table 1.

TABLE 1 Conditions 01 to 11 used for CRU measurements of fuel blends.Delay Pre- p_(Chamb)/ Main bar Pre-Inj. Main Inj. (usec) WorkingT_(Wall)/° C. range p_(Fuel)/bar Period/ Period/usec range EGR/% pointrange 350-590 10-75 range 200-1600 usec 0-1400 range 100-1500 100-3000range 0-100 No. Injections 01 590 30 900 0 900 100 0 10 02 590 50 900 0900 100 0 10 03 590 75 900 0 900 100 0 10 04 560 30 900 0 900 100 0 1005 560 50 900 0 900 100 0 10 06 560 75 900 0 900 100 0 10 07 530 30 9000 900 100 0 10 08 570 21.4 200 0 1500 100 0 10 09 530 75 900 0 900 100 010 10 530 50 900 0 900 100 0 10 11 590 65 1600 0 1500 100 0 10

Ignition Delay

The primary data obtained from CRU measurements are pressure-timetraces, from which the ignition delay (ID), burn period (BP) and maximumpressure increase (MPI) can be determined.

There are different definitions for ignition delay. Typically, ID^(5%)or ID^(0.2) measurements are used, which are defined as the time takenfor the pressure in the combustion chamber to rise to its initial valueplus 5% of the MPI, or to 0.2 bar above its initial value, respectively.In these studies both values are used, although ID^(0.2) is preferreddue to the lower standard deviations observed.

A derived ignition quality (DIQ) is obtained from the ignition delay viacomparison with primary reference fuels (PRFs), which have a knowncetane number. Since ignition delay correlates with the cetane number ofthe fuel, a calibration model can be obtained for all conditions. Withsuch calibrations, DIQ can be determined for any unknown fuel andderived as a pseudo-cetane number.

The PRFs used in this study are shown in Table 2.

TABLE 2 PRFs used to determine the ID-DIQ calibration models. PRF CetaneHeptamethylnonane CN number (Vol %) (Vol %) (by definition) 1 40.0 60.049 2 100.0 0.0 100 3 64.7 35.3 70 4 88.2 11.8 90 5 52.9 47.1 60 6 76.523.5 80 7 35.3 64.7 45

The effect of the different organic nitrates in fuel blends was comparedby measuring ignition delay and calculating the percentage ID-reductionrelative to the respective base fuel. The results for exemplarycombustion conditions 03, 05, 07 and 08 are illustrated in FIG. 2.

The graphs show the data obtained at organic nitrate concentrations of1.0% w/w (left, graphs A, C, E and G) and 0.1% w/w (right, graphs B, D,F and H) at representative combustion conditions (first row, conditiona03, graphs A and B; second row, condition a05, graphs C and D; thirdrow, condition a07, graphs E and F; and fourth row, condition a08,graphs G and H). Data for organic nitrates is as follows: exo bornylnitrate (column 1); menthly nitrate (column 2); oleyl nitrate (column3); 1,10-decyl dinitrate (column 4); 1-octadecyl nitrate (column 5);nitro-substituted methyl oleate (column 6); cholesterol nitrate (column7); 1-octyl nitrate (column 8); 1-tetradecyl nitrate (column 9);2-ethylhexyl nitrate (positive control, column 10); 1-hexadecyl nitrate(column 11); dodecyl/tridecyl nitrate mixture (column 12);nitro-substituted ethyl abietate (column 13); and exo fenchyl nitrate(column 14)

These data show that all of the organic nitrates provided a benefitcompared to the base fuel by achieving a shorter ignition delay. Ingeneral, menthyl nitrate achieved a greater reduction in ignition delaythan the known cetane enhancer, 2-EHN, under all test conditions, andwas the most effective combustion enhancer tested. Under some testconditions exo-fenchyl nitrate or 1,10-decyl dinitrate gave the greatestreductions in ignition delay. In general, 1,10-decyl dinitrate, neodo123nitrate (mixture of dodecyl and tridecyl nitrates), and 1-octyl nitrateachieved similar reductions in ignition delay to 2-EHN under all testconditions. Some organic nitrates displayed a concentration-dependenteffect. For example, exo-fenchyl nitrate was generally the mosteffective cetane enhancer at low concentrations (e.g. 0.1% w/), while itwas generally slightly less effective than menthyl nitrate at higherconcentrations (e.g. 1.0% w/w). Furthermore, the effect was mostpronounced under the milder test conditions. Interestingly, the fattyacid derived nitrates displayed slight reductions in effectiveness aschain length increased. Of the saturated and unsaturated nitrates of thesame carbon-chain length, it appears that the saturated nitrates providea stronger cetane boost than unsaturated molecules. The results alsosuggest that the nitrated molecules are more effective cetane enhancersthan the nitro molecules.

Dose Rate Response

The effectiveness of the terpene nitrates, exo-bornyl nitrate andL-menthyl nitrate, were tested at different dose levels (i.e. 0.05% w/w,0.1% w/w, 0.5% w/w and 1.0% w/w). The results (not shown) illustratethat there is a saturation effect for cetane enhancement. Thus, at thelow concentrations tested (0.05% w/w and 0.1% w/w) there was a clearconcentration-dependent reduction in ignition delay. However, arelatively small additional reduction in ignition delay was observed inthe concentration range of 0.1% w/w to 0.5% w/w and 1.0% w/w. Thissaturation effect was found under all combustion conditions tested.

Importantly, therefore, the effective additives can be used to goodeffect at low concentrations, which is useful in many respects, such asproduction level, storage, cost, handling and safety.

Derived Ignition Quality (DIQ)

The derived ignition quality (DIQ) was determined for each of theorganic nitrates at concentrations of 0.1% w/w and 1.0% w/w.

By way of example, the DIQ for the diesel base fuel comprisingexo-fenchyl nitrate at different concentrations (i.e. 0.1% w/w and 1.0%w/w) was determined relative to the non-modified base fuel compositionand relative to blends with the known cetane enhancer, 2-EHN. Studieswere carried out under all 11 sets of combustion conditions and theresults are shown in FIG. 3. The left graph shows the results of a basefuel containing exo-fenchol nitrate at concentration of 0.1% w/w, andthe right graph shows the results for concentration of 1.0% w/w,measured under all test combustion conditions, a01 to a08 (columns 1 to8, respectively) and a09 to all (columns 10 to 12, respectively). Ineach graph, for comparative purposes the DIQ of the diesel base fuelunder reaction condition a08 is illustrated (column 9);

By way of example, these data show that at IQT-conditions 08, asspecified in cetane number measurement according to EN15195 (Oxley etal. (2000) Energy Fuels, 14, 1252-1264), the 0.1% w/w exo-fenchylnitrate fuel blend achieved a DIQ of 52.2, while the 1.0% w/wexo-fenchyl nitrate fuel blend had a DIQ of 63.7. This represents asignificant increase on the DIQ of 44.1 for the base fuel compositionalone. These results thus show a cetane number enhancement of 7.1 at1000 ppm, which is similar to that expected for 2-EHN.

Isothermal and Isobaric Response

The isothermal and isobaric responses of the fuel blends were measuredand compared to see the effect of temperature and pressure on ignitiondelay for each organic nitrate. In particular, the experimental designthat was chosen for CRU measurements allowed for isobaric and isothermalcomparison of the additives, which provided an opportunity to follow theresponse of additives as combustion conditions changed from mild toharsh.

The data, not illustrated, demonstrated that in general all additivesshowed the same response pattern irrespective of their specificperformance as ignition enhancers. Higher temperature and higherpressure (harsh conditions) resulted in reduced ignition delays (i.e.each additive demonstrated a parallel down-shift compared to the basefuel as pressure and/or temperature increased. Thus, under combustioncondition 11 (maximum power), typically, the ignition delay was theshortest. This may be due to faster injection rate and mixing. However,it was found that the relative difference in ignition delay incomparison to the base fuel increased with reducing temperature. Inother words, relative to base fuel the reduction in ignition delay isgreater at mild conditions.

Thermal Stability

Differential scanning calorimetry/thermogravimetric analysis (DSC/TGA)was used to evaluate the thermal stabilities of the nitrates and othercompounds prepared in the study; and also to provide information on thedecomposition mechanism from mass spectrometry (MS) of the volatileproducts.

The experiments were carried out under argon flow, with a temperatureprofile of 5° C./min up to 200-350° C. Assays were carried out underatmospheric pressure and, hence, the thermal degradation of productsthat evaporate prior to thermal decomposition could not be studied (e.g.1-octyl nitrate).

An exemplary DSC/TGA assay trace for 1,10-decyl dinitrate is illustratedin FIG. 5. As illustrated, the product begins to decompose at atemperature of approximately 170° C. The line beginning in the top leftof the graph illustrates the weight loss of the product ondecomposition. The two traces showing sharp peaks towards the right handside of the graph represent the exotherm (right-hand peak: maximum atabout 200° C.) and the concentration of the decomposition product(left-hand peak).

Typically several ions are observed in MS studies of the organicnitrates under thermal decomposition (e.g. M (or M/2)=12, 14, 18, 28,30, 44, 46 and 56).

Without wishing to be bound by theory, the rate-determining step in thedecomposition of primary and secondary nitrates is thought to be thehomolysis of the RO—NO₂ bond to give the alkoxyl radical and NO₂. Thismay be followed by one or more competing reactions of the alkoxylradical, e.g.: 13-cleavage of the alkoxyl radical to liberateformaldehyde (CH₂O) and form an alkyl radical, which may undergoβ-cleavage or other reaction; loss of a hydrogen radical (via thealternative-cleavage) to give an anhydride; and hydrogen abstractionfrom a suitable hydrogen donor (e.g. PhCH₂R) to give the alcohol.

The degradation mechanism and product distribution for a particularnitrate depends on whether it occurs in the liquid or gas phase and thepresence of other components. Without being bound by theory, it isexpected that the compound 1,10-decyl dinitrate would decompose in asimilar fashion to the mononitrates, except perhaps, more quickly as thetwo nitrate functionalities are effectively isolated.

The DSC/TGA method was initially used to confirm that product sampleswere sufficiently stable to allow safe transportation and storage.However, the methods also provide detailed information on the thermaldegradation of potential cetane enhancers, including on the mechanism ofdecomposition, at least for those products that do not (partially)evaporate below the degradation temperature (linear organic nitrateshaving less than 14 carbon atoms may require a modified method).

The DSC method provides some qualitative information on the relativestability of different cetane enhancers, but, as it involves atemperature scan, it does not directly reflect the relative thermalstability at any particular temperature. Hence, the stability orderderived from the temperature of exotherm maximum may differ from thatdetermined at a particular temperature in kinetic experiments if theactivation energy of the compounds differs. Components exhibiting abroad exotherm would be expected to have a lower activation energy thanthose with a narrow exotherm.

Methods are known for determining kinetic parameters from single scanDSC heat flow data, from the dependence of the temperature of maximumexotherm on the heating rate, and using isothermal DSC methods. Forexample, the activation energy and pre-exponential factor for thedecomposition of di-t-butylperoxide have been determined from DSC. Itshould, in principle, be possible (e.g. using adiabatic measurements) toextend these methods for the determination of Arrhenius kineticconstants to cetane enhancers based on organic nitrates and relatedcompounds.

The identification of more stable cetane enhancers is an attractiveprospect as it may then be possible to transport and supply such cetaneenhancers in more concentrated stocks than is possible for the currentcommonly used cetane enhancers, such as 2-EHN. This may then provide anumber of advantages over existing formulations, such as by reducing thevolume of chemicals that need to be added to fuels and, thus,transported and stored. It may also reduce or eliminate the need forsome specialist storage, transportation and/or handling equipment toavoid combustion hazards. While the organic nitrates of the invention ingeneral are more stable than some known cetane enhancers (e.g.Di-tert-butylperoxide, DTBP), tetradecyl nitrate is a particularlyattractive cetane enhancer of the invention that appears to be morestable than 2-EHN.

Adiabatic Thermal Stability Measurements

To obtain a quantitative measurement of the thermal stability of somecetane enhancers adiabatic heat-wait search (HWS) experiments werecarried out using a Phi-Tech calorimeter, which gives an accuratedetermination of kinetic and thermodynamic parameters of the moleculesdecomposition reaction.

The Phi-Tec equipment is usually operated in a heat-wait-search mode,which means that the temperature of the reactor is increased stepwiseuntil the decomposition onset temperature (DOT) of the reaction isdetected. Onset temperatures are graphically derived by searching forthe temperature at which deviation from linear temperature rise takesplace due to self-heating of the sample. The apparatus thanautomatically tracks the runaway exotherm to 490° C. maximum (at shortexposure time; otherwise 400° C.). The decomposition onset is detectedwhen the self heating rate exceeds 0.02° C./min.

Typical Phi-Tec reaction conditions:

Intake sample: ca. 66 g;Test cell material: SS-316 (STRCA type, 1*⅛″ tube connection);Void cell volume: ca. 110 ml;Start temperature: ca 20° C. below T_(onset) (ex thermal screening/TSu);Detection limit: 0.02° C./min;

Step: 5 to 20° C.;

Maximum search temperature: 270° C.;Maximum track temperature: 300° C.;Maximum pressure: 80 bara.

The organic nitrates tetradecyl nitrate (TDN), menthyl nitrate (MN)—both of the invention, 2-Ethylhexylnitrate (2-EHN; Aldrich), anddi-tert-butylperoxide (DTBP; Merck)—known in the prior art were assessedin consecutive experiments. Cetane enhancers were diluted to approx. 15%w/w in toluene for thermal measurements. Tetradecyl nitrate was alsoblended at approx. 27% w/w for PhiTec measurements due to its highmolecular weight. Experiments with DTBP were conducted in an inertatmosphere (under N₂), which is expected to increase the onsettemperature compared to an oxygen-containing atmosphere. Otherexperiments were carried out under air to mimic typical conditions inuse. The data obtained from Phi-Tec measurements are summarised in Table3.

The actual start temperature at which the Phi-Tec switched to adiabatictracking mode (i.e. first detection of decomposition onset temperature,DOT) is the temperature of the sample at which the self-heating rateexceeds 0.02° C./min. From the final tracking temperature, the adiabatictemperature rise can be calculated by multiplication with Phi, the heatdistribution factor for the reaction (as is known by the skilled personin the art). The variable heat capacity (Cp) for the novel nitrates isassumed to be 2.54 J/gK.

As indicated in Table 3, in these experiments MN was found to beslightly less stable than 2-EHN, which correlates with its effectivenessas a cetane enhancer described above. Overall, TDN was found to be themost stable molecule, such that the stability of the compounds was inthe order DTBP<<MN<EHN<<TDN.

Since the maximum temperature rise is within the Phi-Tec operatingwindow (i.e. below 400° C.), 1^(St) order kinetic data can be obtainedfrom the exothermic decomposition reaction by plotting in an Arrheniusplot ln((dT/dt)/60)/(T−T_(max))) against 1/T. The frequency factor k(intercept) and activation energy Ea (slope) can thus be derived. It isreasonable to assume the decomposition reaction as 1^(st) order, as itis solely dependent on the concentration of the nitrate or peroxiderespectively, which approach has been documented by other investigators(Oxley et al. (2000) Energy & Fuels, 14, 1252).

TABLE 3 Kinetic and thermodynamic data derived from Phi-Tec HWSexperiments Phi-tec Conc. Ea Onset Compound (w %) Atmos. Phi (kJ/mol)Temp. (° C.) DTBP 15.0 N₂ 1.10 160.2 111 DTBP 14.6 N₂ 1.10 162.0 110DTBP 14.9 N₂ 1.10 157.6 110 DTBP 14.9 N₂ 1.10 158.4 112 2-EHN 15.2 air1.10 178.2 132 2-EHN 14.9 air 1.10 177.5 135 TDN 15.1 air 1.10 190.3 142TDN 27.2 air 1.10 179.1 136 TDN 27.3 air 1.11 179.5 137 MN 14.9 air 1.10171.1 123 MN 14.9 air 1.10 171.1 127

As illustrated in the data, the ranking described for thermal stabilityof the compounds is also reflected in the activation energies (Ea),which indicates that compounds with lower reaction onset temperaturealso exhibit lower activation energy. The reaction enthalpies (notshown) for the respective compounds correlate with the adiabatictemperature rise. DTBP demonstrated the lowest reaction enthalpy and thelowest temperature rise, whereas the organic nitrates compared well,with both the reaction enthalpy and the adiabatic temperature rise arein the same order of magnitude.

CONCLUSIONS

The synthesis of possible alternative embodiments of cetane enhancers tothe commonly used 2-ethylhexylnitrate (2-EHN) was investigated, with thefocus on the use of renewable feedstocks. There are some safety concernssurrounding the production, transport and use of this current compound.Furthermore, 2-EHN functions most effectively under mild engineconditions and a cetane enhancer that also works well under harsherengine conditions is desirable.

Potential cetane enhancers were prepared via the nitration of variousbio-feedstocks: terpene alcohols—borneol, fenchol and menthol (andpinene); fatty alcohols; unsaturated FAME's; 1,10-Decandiol, ultimatelyderived from ricinoleic acid (castor oil); and ethyl abietate, a resinester derived from tall oil.

Following determination of the optimum experimental conditions fornitration in small-scale experiments, different nitrated products wereprepared in 25-50 g scale. The majority of the samples were well-definednitrates (R—ONO₂), prepared from alcohol precursor by reaction withnitric acid, optionally in combination with other reagents. Treatment ofthe two olefin feedstocks with dinitrogen tetroxide (N₂O₄) led tomixtures of compounds. All samples were characterised by NMR and IRspectroscopy. DSC/TGA analyses was used to provide information on theexothermic decomposition of the products. As expected, most nitratesunderwent exothermic decomposition in a narrow temperature range, around210° C., whereas products derived from olefins decomposed over a muchbroader temperature range.

Following confirmation (from DSC/TGA and NMR monitoring of storagestability) that the samples were sufficiently stable to transport,products were shipped for evaluation of their effectiveness as cetaneenhancers in fuel compositions at a Combustion Research Unit.

A thermal screening and calorimetric assessment of the organic nitratesfor use as cetane enhancers has been conducted for comparison withcetane enhancers known in the art (e.g. 2-EHN and DTBP).

Further modifications and alternative embodiments of various aspects ofthe invention will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

1. A diesel fuel composition for use in a compression ignition engine, said diesel fuel composition comprises an organic nitrate selected from the group consisting of: a cyclic nitrate of Formula (4):

and Formula (4A):

wherein each of R₁ to R₉ is independently selected from H or C₁-C₆ alkyl, or nitrate(—ONO₂), wherein optionally one of R₄ and R₅ forms an optionally substituted alkylene bridge with one of R₈ and R₉, which may be substituted by one or more C₁-C₆ alkyl, and/or nitrate(—ONO₂); wherein at least one of R₁ to R₉ is not H, and provided that no more than one R₂ to R₉ comprises a nitrate group.
 2. The diesel fuel composition of claim 1, wherein at least one of R₁ to R₉ is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl.
 3. The diesel fuel composition of claim 1, wherein one of R₄ and R₅ is methyl and one of R₈ and R₉ is isopropyl.
 4. The diesel fuel composition of claim 1, wherein one of R₄ and R₅ forms an optionally substituted alkylene bridge with one of R₈ and R₉; and wherein the alkylene bridge has the formula —(CR_(a)R_(b))_(n)—, wherein R_(a) and R_(b) are independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or
 2. 5. The diesel fuel composition of claim 4, wherein the cetane number improver is defined by Formula (5):

or Formula (5A):

wherein (i) at least two of R_(a), R_(b), R₂, R₃ and R₉ are methyl; (ii) R_(a) and R_(b) are methyl; (iii) R₂ and R₃ are methyl; (iv) R_(a), R_(b) and R₉ are methyl; or (iv) R₂, R₃ and R₉ are methyl.
 6. The diesel fuel composition of claim 1, wherein the cetane number of the diesel fuel composition containing the organic nitrate is higher than the cetane number of the diesel fuel composition lacking the organic nitrate.
 7. The diesel fuel composition of claim 1, wherein said diesel fuel composition has a cetane number of between 52 and
 58. 8. The diesel fuel composition of claim 1, wherein the diesel fuel composition comprises one or more additional organic nitrate.
 9. A method for reducing the ignition delay and/or increasing the cetane number of a diesel fuel composition, said method comprises adding to the composition an amount of an organic nitrate, wherein the organic nitrate is selected from the group consisting of: a cyclic nitrate of Formula (4):

and Formula (4A):

wherein each of R₁ to R₉ is independently selected from H or C₁-C₆ alkyl, or nitrate(—ONO₂), wherein optionally one of R₄ and R₅ forms an optionally substituted alkylene bridge with one of R₈ and R₉, which may be substituted by one or more C₁-C₆ alkyl, and/or nitrate(—ONO₂); wherein at least one of R₁ to R₉ is not H, and provided that no more than one R₂ to R₉ comprises a nitrate group.
 10. The method of claim 9, wherein at least one of R₁ to R₉ is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl.
 11. The method of claim 9, wherein one of R₄ and R₅ is methyl and one of R₈ and R₉ is isopropyl.
 12. The method of claim 9, wherein the cetane number improver is defined by Formula (5):

or Formula (5A):

wherein (i) at least two of R_(a), R_(b), R₂, R₃ and R₉ are methyl; (ii) R_(a) and R_(b) are methyl; (iii) R₂ and R₃ are methyl; (iv) R_(a), R_(b) and R₉ are methyl; (iii) R₂, R₃ and R₉ are methyl.
 13. The method of claim 9, wherein said method is configured to increase the cetane number of the diesel fuel composition to achieve a target cetane number.
 14. The method of claim 9 further comprising adding one or more additional organic nitrate to the fuel composition.
 15. The method of claim 9, wherein said method is configured to reduce the amount of 2-ethylhexyl nitrate (2-EHN) in the diesel fuel composition to achieve the target cetane number.
 16. The diesel fuel composition of claim 1, wherein the organic nitrate is present in the diesel fuel composition at a concentration of: (a) between 0.025% and 2.0% w/w; (b) between 0.05% and 1.0% w/w; or (c) 0.05% w/w, 0.1% w/w, 0.5% w/w or 1.0% w/w; based on the total weight of the fuel composition.
 17. The diesel fuel composition of claim 1, wherein the organic nitrate is further defined as follows: (i) R₁, R₆ and R₇ are H; (ii) R₁, R₆ and R₇ are H; R₂ and R₃ are methyl; (iii) R₁, R₆ and R₇ are H; R₅ is H and R₉ is methyl; or (iv) R₁, R₆ and R₇ are H; R₂ and R₃ are methyl; R₅ is H and R₉ is methyl.
 18. The diesel fuel composition of claim 1, wherein the organic nitrate is selected from the group consisting of: bornyl nitrate, fenchyl nitrate, and menthly nitrate. 